Reference signal generating configuration for an interferometric miniature grating encoder readhead using fiber optic receiver channels

ABSTRACT

A reference mark configuration for an interferometric miniature grating encoder readhead using fiber optic receiver channels is provided. The readhead may include “primary” fibers that provide reference mark primary signals processed to generate a reference signal with accuracy of approximately 0.2 microns. In some embodiments, a zone grating type reference mark may be embedded in a periodic scale grating, and configured such that it provides strong reference mark primary signals without disrupting periodic incremental measurement signals associated with the periodic scale grating. In one embodiment, the readhead may include “secondary” fibers used to generate reference mark secondary signals processed to generate a reference signal with accuracy of approximately 20 nanometers.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/275,170, filed Nov. 20, 2008, which is acontinuation-in-part of U.S. patent application Ser. No. 11/782,608,filed Jul. 24, 2007, the disclosures of which are hereby incorporated byreference herein.

FIELD OF THE INVENTION

This invention relates generally to displacement sensing opticalencoders, and more particularly to providing a reference signal for aminiature fiber optic encoder utilizing optical fibers as receiverelements.

BACKGROUND OF THE INVENTION

Various miniature fiber optic grating encoders that use fiber opticreceiver channels are known, including those disclosed in U.S. Pat. Nos.6,906,315; 7,053,362; and 7,126,696 (the '315, '362, and '696 patents),each of which are hereby incorporated herein by reference in theirentirety. Such miniature encoders offer a desirable combination offeatures, which may include extremely small size, very high accuracy,electrical noise immunity, and very high speed operation.

Many motion control and/or position measurement systems, or the like,include provisions for inputting a reference signal that is usable toidentify a particular period within a grating scale. The referencesignal, generally corresponding to a feature that is fixed relative tothe grating scale, provides a reference point that eliminates theposition ambiguities that may otherwise arise in incremental typedisplacement measuring systems, which count signal periods as a basisfor long range measurements.

However, a reference signal generating configuration that is easily andeconomically combinable with miniature fiber optic grating encoders,such as those included in the above references, and that providessimilar desirable features, is not known. Such a reference signalgenerating configuration would be desirable.

SUMMARY OF THE INVENTION

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

Briefly, the present invention is directed to providing a miniaturefiber optic readhead and scale arrangement for measuring displacementthat also includes a miniature fiber optic reference signal generatingconfiguration usable to provide a reference position indication. Invarious embodiments, the scale includes a scale track comprising a firsttype of track portion providing first level of zero order reflectance(e.g., a phase grating), and a reference mark providing a second levelof zero order reflectance (e.g., a mirror). Respective fiber opticreference signal receiver channels include respective apertures thatreceive detectably different amounts of zero order reflected lightdepending on their proximity and/or overlap with the reference mark, anda relationship between the optical reference mark signals is indicativeof the reference position. In some embodiments or configurations, areference mark according to this invention has length or boundaryspacing dimensions along the measuring axis direction that aredetermined based on certain fiber optic reference signal receiverchannel aperture dimensions (e.g., size and spacing) in the readhead,which establishes a desirable relationship between the resultingrespective reference mark signals.

In some configurations a fiber optic readhead and scale track thatprovides the reference position indication is separate from a fiberoptic readhead and scale track that provides periodic incrementalmeasurement signals. In some configurations an integrated fiber opticreadhead and an integrated scale track provide both the referenceposition indication and the periodic incremental measurement signals.

In some embodiments or configurations, respective fiber optic referencesignal receiver channels that include respective apertures are used togenerate higher resolution reference mark secondary signals and thepreviously mentioned optical reference mark signals are used as lowerresolution reference mark primary signals. In particular, geometricdesign rules (e.g., for size and spacing) are used to configure spatialfilter masks for the secondary signal receiver channel apertures suchthat they provide two spatially periodic secondary signals correspondingto interference fringes received outside of the receiving area of theprimary signal receiver channels. A spatial frequency and/or rate ofchange of the reference mark secondary signals may be several timeshigher than an effective spatial frequency and/or rate of change of thereference mark primary signals. In some embodiments, the reference markprimary signals transition from a high to a low value over a distance onthe order of 100 microns. In some embodiments, the spatial frequency ofthe secondary reference mark signals is on the order of 2-4 microns, andthey are 180 degrees out of phase with one another. A “secondary”crossing point of the reference mark secondary signals can be identifiedthat is spatially adjacent to the “primary” crossing point of thereference mark primary signals (which is the primary reference markposition). In some embodiments, the primary crossing point may be usedto identify the primary reference mark position with a resolution andrepeatability on the order of 0.2 microns. The secondary crossing pointmay be used to identify a secondary reference mark position with aresolution and repeatability on the order of 20 nanometers (10×improvement) due to the higher spatial frequency and/or higher rate ofsignal change of the reference mark secondary signals.

In some embodiments or configurations, particularly where an integratedfiber optic readhead and an integrated scale track provide both thereference position indication and the periodic incremental measurementsignals, the reference mark may comprise a novel zone grating referencemark structure embedded in a periodic scale grating track (e.g., a phasegrating) that provides the incremental measurement signals. In variousembodiments, the zones of the novel zone grating reference mark maycomprise alternating raised and recessed reflective phase gratingelements that have different widths WEZ and WGZ along the measuring axisdirection (such that a “duty” cycle of the phase grating elements in thezone grating reference mark is different from the approximately 50:50duty cycle of the surrounding periodic scale grating). The difference inthe grating elements widths may be “reversed” (that is the duty cyclemay be reversed) between adjacent zones of the zone grating referencemark, such that each zone nominally provides a component of reflectedlight having a phase that is opposite to that of the light from theadjacent zones. As a result, a zone grating reference mark can create afocusing effect for a portion of the light reflected from the zonegrating reference mark. To the extent that the widths WEZ and WGZ match,the matching portions may suppress a certain amount of zero orderreflected light, and contribute more energy to odd-order diffractedlight, similar to the scale grating. Thus, the novel zone gratingreference mark may partially contribute to interference fringes thatprovide the incremental measurement signals, and avoid significantdisruption of such signals. Conversely, to the extent that the widthsWEZ and WGZ are unmatched, the unmatched portions may contribute “zeroorder” reflected light. The behavior of such light may be understood interms of the well known Huygens-Fresnel principle. In particular, it maybe understood that the unmatched portions of the widths WEZ and WGZ maycontribute a light component that behaves according to known zone plateprinciples. Thus, the novel zone grating reference mark may contributelight from various zones that constructively and destructively combinesat certain locations such that it, in effect, provides a concentrated orfocused scale light component in a zone grating reference mark signaleffect region. The balance between the strength of light component fromto zone grating reference mark that contributes to the interferencefringes that provide the incremental measurement signals, relative tothe light component that contributes to the zone grating reference marksignal effect region, may be adjusted by adjusting the duty cycle of thewidths WEZ and WGZ, as described in further detail with reference to thedrawings.

The zone grating reference mark signal effect region may generally benarrower along the measuring axis direction than the zone gratingreference mark. Respective fiber optic reference signal receiverchannels include respective apertures that receive detectably differentamounts of reference mark signal light depending on their proximityand/or overlap with the reference mark signal effect region, and arelationship between the optical reference mark signals is indicative ofthe reference position. In various embodiments or configurations, a zonegrating reference mark is designed such that the corresponding zonegrating reference mark signal effect region has length or boundaryspacing dimensions along the measuring axis direction that aredetermined based on certain fiber optic reference signal receiverchannel aperture dimensions (e.g., size and spacing) in the readhead,which establishes a desirable relationship between the resultingrespective reference mark signals.

Importantly, a fiber optic reference signal generating configurationaccording to this invention offers desirable features similar to thoseof known miniature fiber optic grating encoders that provide incrementalmeasurement (e.g., those disclosed in the '696 patent). For example,importantly, a fiber optic reference signal generating configurationaccording to this invention can be used at operating gaps similar oridentical to the interferometric type fiber optic encoders disclosed inthe '696 patent. In addition it offers similar extremely small size,high accuracy, electrical noise immunity, and very high speed operation.A miniature fiber optic reference signal generating configurationaccording to this invention is thus readily and economically combinablewith desirable high-accuracy miniature fiber optic incrementalmeasurement encoders.

Hence, the invention overcomes the disadvantages of prior art opticaldisplacement sensing devices and provides new application possibilitieswith an ultra-compact, highly accurate, economical and high speedconfiguration.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is an isometric view of first embodiment of a miniature fiberoptic readhead and scale arrangement that includes a reference signalgenerating configuration according to this invention;

FIG. 2 is an isometric view of a second embodiment of a miniature fiberoptic readhead and scale arrangement that includes a reference signalgenerating configuration according to this invention;

FIG. 3 is an isometric view of a third embodiment of a miniature fiberoptic readhead and scale arrangement that includes a reference signalgenerating configuration according to this invention;

FIG. 4 is an isometric view of a fourth embodiment of a miniature fiberoptic readhead and scale arrangement that includes a reference signalgenerating configuration according to this invention;

FIG. 5 is an isometric view showing one exemplary embodiment of agrating and reference mark structure according to this invention;

FIGS. 6A and 6B are isometric views schematically showing variousaspects of a first embodiment of a reference signal generatingconfiguration according to this invention;

FIG. 7 is an isometric view schematically showing a portion of a secondembodiment of a reference signal generating configuration according tothis invention;

FIG. 8 is a diagram showing the reference signals generated according tothe reference signal generating configurations of FIGS. 6A, 6B, and FIG.7;

FIG. 9 is an isometric view schematically showing various aspects of theoperation of a first integrated reference signal and incremental signalgenerating configuration according to this invention, including a thirdembodiment of a reference signal generating configuration according tothis invention;

FIG. 10 is an isometric view showing a portion of the integratedreference signal and incremental signal generating configuration shownin FIG. 9, including additional details;

FIG. 11 is an isometric view showing a portion of a second integratedreference signal and incremental signal generating configurationaccording to this invention, including a fourth embodiment of areference signal generating configuration according to this invention;

FIG. 12 is a diagram schematically showing the reference signalsgenerated according to the integrated reference signal and incrementalsignal generating configurations of FIGS. 10, and 11;

FIG. 13 is an isometric view showing a portion of a fifth embodiment ofa reference signal generating configuration according to this invention;

FIG. 14 is a diagram showing the reference signals generated accordingto the reference signal generating configuration of FIG. 13;

FIG. 15 is an isometric view showing a portion of a sixth embodiment ofa reference signal generating configuration according to this invention;

FIG. 16 is an isometric view showing a portion of a seventh embodimentof a reference signal generating configuration according to thisinvention;

FIGS. 17A and 17B are illustrations showing alternative aperture maskconfigurations usable in place of portions of an aperture maskconfiguration shown in FIG. 16;

FIG. 18 is an isometric view showing a portion of an eighth embodimentof a reference signal generating configuration according to thisinvention;

FIG. 19 is a diagram showing various signal relationships which may beassociated with primary and secondary reference signals according tothis invention;

FIGS. 20A and 20B are illustrations showing alternative reference markstructures usable in place of the reference mark structure shown in FIG.5;

FIG. 21 is an isometric view of an integrated miniature fiber opticreadhead and scale arrangement according to this invention, whichincludes an integrated incremental signal and reference signalgenerating configuration;

FIG. 22 is an isometric view showing a generic integrated scale gratingand zone grating reference mark structure usable in various embodimentsaccording to this invention;

FIG. 23 is an isometric view schematically showing various aspects of anintegrated incremental signal and reference signal generatingconfiguration, which includes an integrated scale grating and zonegrating reference mark arrangement usable in various embodimentsaccording to this invention;

FIG. 24 is an isometric view schematically showing various additionalaspects of integrated incremental signal and reference signal generatingconfiguration of FIG. 23;

FIG. 25 is a diagram showing the reference signals generated accordingto the integrated incremental signal and reference signal generatingconfiguration of FIGS. 23 and 24;

FIG. 26 is a diagram showing a schematic side view showing variousaspects of the operation and design of an integrated scale grating andzone grating reference mark arrangement usable in various embodimentsaccording to this invention;

FIGS. 27A and 27B are illustrations showing alternative aperture maskconfigurations usable in place of portions of the aperture maskconfiguration shown in FIG. 24; and

FIG. 28 is an isometric view schematically showing various aspects of anintegrated incremental signal and reference signal generatingconfiguration usable in various embodiments according to this invention,which uses an integrated scale grating and zone grating reference markarrangement that includes two zone grating reference mark portions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is an isometric view of a first embodiment of a miniature fiberoptic readhead and scale arrangement 1000 that includes a referencesignal generating configuration according to this invention. As shown inFIG. 1, the miniature fiber optic readhead and scale arrangement 1000includes a scale member 81 that includes a scale grating 80, anincremental readhead 100, and a reference mark readhead 200. It will beappreciated the readheads 100 and 200 will generally be mounted rigidlyrelative to one another, or formed as a single unit, so that theirdisplacements are synchronized.

An orthogonal XYZ coordinate system may be defined such that the Y-axisis parallel to the bars of the scale grating 80, the Z-axis is normal tothe surface of the scale grating 80, and the X-axis is orthogonal to theY-Z plane. A measuring axis 82 is parallel to the X-axis. In operation,the scale member 81 displaces along the measuring axis 82 such that thereadhead 100 is displaced along an incremental measuring scale track 86that includes the scale grating 80, and the readhead 200 is displacedalong a reference scale track 88. In FIG. 1, an approximate boundarybetween the incremental measuring scale track 86 and reference scaletrack 88 is indicated by a dashed line 10. In the embodiment shown inFIG. 1, the reference scale track 88 generally includes the scalegrating 80. However, importantly, the reference scale track 88 alsoincludes one or more reference mark zones 251, described in greaterdetail below.

The incremental readhead 100 may be a prior art miniature fiber opticreadhead that comprises a ferrule 101 that houses and positions the endsof a plurality of optical fibers 130 that are included in a fiber-opticcable 195. In various embodiments, the incremental readhead 100 maycomprise any of the types of incremental readheads described in theincorporated references. In the embodiment shown in FIG. 1, theincremental readhead 100 comprises an interferometric-type readhead,described in detail in the incorporated '696 patent. Briefly, inoperation, the readhead 100 outputs a diverging coherent source light150 from the central one of the optical fibers 130, which illuminatesthe scale grating 80 at an illumination spot 153, where it is reflectedand diffracted to provide scale light 155. In various embodiments, thescale grating 80 is a phase grating configured to suppress zero-orderreflection. Therefore, the scale light 155 comprises primarily+/−first-order diffracted lights that are reflected to the readhead 100.The +/−first-order diffracted lights form a field of interferencefringes proximate to a receiver plane 160 of a phase mask element 161.The phase mask element 161 provides a plurality of spatial filters atthe receiver plane 160, having different spatial phases over the ends ofthe outer optical fibers 130, in order to provide a plurality of fiberoptic incremental measurement signal receiver channels, as described inthe '696 patent. As a result of the spatial filtering, the fiber-opticincremental measurement signal receiver channels may output periodicoptical signals having different spatial phases (e.g., quadraturesignals) when the scale grating 80 is displaced relative to the readhead100.

The reference mark readhead 200 may comprise a ferrule 201 that housesand positions the ends of a plurality of optical fibers 230 that areincluded in a fiber-optic cable 295. In various embodiments, thereference mark readhead 200 may comprise various reference signalgenerating configurations according to this invention, as described ingreater detail below. Briefly, in operation, the readhead 200 outputs adiverging source light 250 from the central one of the optical fibers230, which illuminates the scale grating 80 and/or a reference mark zone251, at an illumination spot 253. In various embodiments, the divergingsource light 250 is advantageously monochromatic and spatially coherent,and may be temporally coherent in some embodiments. In general, thescale grating 80 provides reflected and diffracted scale light thatproduces a field of interference fringes, in the same manner outlinedabove with reference to the readhead 100. However, in variousembodiments, the reference mark readhead 200 includes no phase maskelement. As a result, the ends of the outer optical fibers 230, whichprovide a plurality of fiber optic reference mark signal receiverchannels, simply receive an approximately constant “average” amount oflight from that interference fringe field, regardless of displacement.

As previously indicated, in various embodiments, the scale grating 80 isa phase grating configured to suppress zero-order reflection. Thus, areference mark may be formed by interrupting the structure and/oroperation of the scale grating 80 by using at least one mirror-likereference mark portion in the reference mark zone 251. In such a case,when the reference mark zone 251 is located in the illumination spot253, the minor-like reference mark portion produces a zero-orderreflection that provides a diverging scale light 254, as shown inFIG. 1. As a result, when the readhead 200 is displaced over thereference mark zone 251, the amount of “averaged” fringe light and theamount of zero-order reflected light that is received and transmitted asa reference signal (by any one of the ends of the outer optical fibers230 that is used as a fiber optic reference mark signal receiverchannel) will be modulated as a function of the amount of overlap of theillumination spot 253 and the reference mark portion(s) reference markzone 251. A plurality of respective fiber optic reference mark signalreceiver channels are used to receive and transmit such modulatedoptical reference signals, such that a reference position can beprecisely determined, as described in greater detail below.

The reference mark portion(s) in the reference mark zone 251 may have awidth WY along the Y-axis direction, and provide an arrangement ofboundaries spaced along the direction of measuring axis 82, as describedin greater detail below. The width WY is generally not critical for thereference mark zone 251, or any of the other reference mark zonesdescribed herein, provided that it is sufficient to allow a desiredalignment tolerance for the readhead 200 within the width of thereference mark scale track 88. In various embodiments, proper spacing ofthe boundaries of the reference mark portion(s) included in thereference mark zone 251 along the direction of measuring axis 82 may becritical for providing reliable and robust reference signals, and maygenerally depend on certain dimensions of the configuration of fibersand/or fiber optic reference mark signal receiver channel aperturesprovided in the readhead 200, as described in greater detail below.

FIG. 2 is an isometric view of a second embodiment of a miniature fiberoptic readhead and scale arrangement 2000 that includes a referencesignal generating configuration according to this invention. Theoperation of the miniature fiber optic readhead and scale arrangement2000 is in some respects similar to that of miniature fiber opticreadhead and scale arrangement 1000 of FIG. 1, and similarly numberedcomponents may be similar or identical in form and operation, except asotherwise indicated below. As shown in FIG. 2, the miniature fiber opticreadhead and scale arrangement 2000 includes a scale member 81 thatincludes a scale grating 80, an incremental readhead 100, and areference mark readhead 200. A primary difference between the miniaturefiber optic readhead and scale arrangement 2000 and miniature fiberoptic readhead and scale arrangement 1000 is that the structure ofreference scale track 88′ is different than that of reference scaletrack 88. In particular, at least that portion of the reference scaletrack 88′ that surrounds the reference mark zone 251′comprises a trackportion that provides significant amount of zero order reflectance(e.g., a mirror-like track portion) when illuminated by the divergingsource light. The reference mark zone 251′ is located within that trackportion. In such an embodiment, the reference mark zone 251′ includes atleast one reference mark portion that provides a significantly lessamount of zero order reflectance than the surrounding track portion(e.g., a grating portion designed to suppress zero order reflection). Insome embodiments, grating reference mark portion(s) included in thereference mark zone 251′ may be identical in structure to the scalegrating 80 that extends along the incremental scale track 86. In oneembodiment, a mirror-like track portion may extend approximately theentire length the reference scale track 88′.

Briefly, in operation, the readhead 100 is fixed relative to thereadhead 200 (e.g., by mounting each readhead in the same mountingbracket) and the scale member 81 displaces along the measuring axis 82such that the readhead 100 is displaced along an incremental measuringscale track 86 and the readhead 200 is displaced along the referencescale track 88′. In general, when the illumination spot 253 is locatedalong the reference scale track 88′ at positions proximate to, but notincluding, the reference mark zone 251′ (e.g., positions comparable tothat indicated by the dashed line 15), the minor-like portion thereference scale track 88′ produces a strong zero-order reflection. As aresult, the ends of the outer optical fibers 230, which provide aplurality of fiber optic reference mark signal receiver channels,receive an approximately constant and “large” amount of light from thatzero-order reflection, over a range of displacements.

A reference mark may be formed by interrupting the structure and/oroperation of the minor-like track portion in the reference mark zone251′. For example, a grating-type reference mark configured to suppresszero-order reflection may be located in the reference mark zone 251′. Insuch a case, when the reference mark zone 251′ is located in theillumination spot 253, the grating portion reference mark suppresseszero-order reflection and produces +/−first order reflections asindicated by the diverging dashed lines in the diverging scale light255, shown above the reference mark zone 251′ in FIG. 2. As a result,when the readhead 200 is displaced over the reference mark zone 251′,the amount of zero-order reflected light is significantly reduced. Inparticular, zero order reflection is suppressed, and a significantportion of the reflected light is deflected away from the readhead 200,as +/−first and third order diffracted light. As a result, the lightthat is received and transmitted as a reference signal by any particularone of the ends of the outer optical fibers 230 that is used as a fiberoptic reference mark signal receiver channel, will be modulated as afunction of the amount of overlap of the illumination spot 253 and thereference mark portion(s) in the reference mark zone 251′. A pluralityof respective fiber optic reference mark signal receiver channels areused to receive and transmit such modulated optical reference signals,such that a reference position can be precisely determined, as describedin greater detail below.

As previously indicated, proper spacing of the boundaries of thereference mark portion(s) included in the reference mark zone 251′ alongthe direction of measuring axis 82 may be critical for providingreliable and robust reference signals, and may generally depend oncertain dimensions of the configuration of fibers and/or fiber opticreference mark signal receiver channel apertures provided in thereadhead 200, as described in greater detail below.

FIG. 3 is an isometric view of a third embodiment of a miniature fiberoptic readhead and scale arrangement 3000 that includes a referencesignal generating configuration according to this invention. Theoperation of the miniature fiber optic readhead and scale arrangement3000 is in some respects similar to that of miniature fiber opticreadhead and scale arrangement 1000 of FIG. 1, and similarly numberedcomponents may be similar or identical in form and operation, except asotherwise indicated below. As shown in FIG. 3, the miniature fiber opticreadhead and scale arrangement 3000 includes a scale member 81 that hasa single scale track that includes a scale grating 80 and a referencemark zone 351, and an integrated incremental and reference mark readhead300, also referred to simply as an integrated readhead 300. The scalegrating 80 may be a phase grating configured to suppress zero-orderreflection. The reference mark zone 351 may include at least onemirror-like reference mark portion, as previously outlined withreference to the reference mark zone 251 of FIG. 1. The integratedreadhead 300 comprises a ferrule 101 that houses and positions the endsof a plurality of optical fibers 330 that are included in a fiber-opticcable 395. In various embodiments, the integrated readhead 300 maycomprise any of the types of integrated readhead configurationsdescribed below with reference to FIGS. 10, 11, and 12, or the like.

Briefly, in operation, the integrated readhead 300 outputs a divergingsource light 350 from the central one of the optical fibers 330, whichilluminates the scale grating 80 at an illumination spot 353. In variousembodiments, the source light 350 is advantageously monochromatic andspatially coherent, and may be temporally coherent in some embodiments.The source light 350 is generally reflected and diffracted to providescale light 355. Scale light 355 comprises +/−first-order diffractedlights that are reflected to the readhead 300, to form a field ofinterference fringes proximate to a receiver plane 360 of a phase maskelement 361, which spatially filters the interference fringes usingphase mask portions having different spatial phases over the ends ofcertain ones of the outer optical fibers 330, in order to provide aplurality of fiber optic incremental measurement signal receiverchannels according to previously described principles. As a result ofthe spatial filtering, certain fiber-optic receiver channels of theintegrated readhead 300 provide incremental measurement signal receiverchannels that may output periodic optical signals having differentspatial phases (e.g., quadrature signals) when the scale grating 80 isdisplaced relative to the readhead 300.

In the embodiment shown in FIG. 3, the phase mask element 361 of theintegrated readhead 300 also includes regions that provide no spatialfiltering over the ends of certain ones of the outer optical fibers 330,to provide a plurality of fiber optic reference mark signal receiverchannels that are used for providing reference signals arising from theminor-like reference mark portion(s) in the reference mark zone 351. Invarious embodiments, when the reference mark zone 351 is located in theillumination spot 353, the minor-like reference mark portion(s) producesa zero-order reflection that provides a diverging scale light 354, asshown in FIG. 3. As a result, when the readhead 300 is displaced overthe reference mark zone 351, the amount of “averaged” fringe light andthe amount of zero-order reflected light that is received andtransmitted as a reference signal by the ends of certain outer opticalfibers 330 that have no spatial filtering and that provide a pluralityof fiber optic reference mark signal receiver channels, will bemodulated as a function of the amount of overlap of the illuminationspot 353 and the reference mark zone 351. The plurality of respectivefiber optic reference mark signal receiver channels are used to receiveand transmit such modulated optical reference signals, such that areference position can be precisely determined, as described in greaterdetail below.

It should be appreciated that it is desirable for the integratedreadhead 300 to continue to output the periodic optical signals havingdifferent spatial phases that are used for incremental displacementmeasurement (e.g., the incremental measurement quadrature signals),simultaneously with outputting the reference mark optical signals whenillumination spot 353 of the integrated 300 overlaps the reference markzone 351. Therefore, in various exemplary embodiments, it isadvantageous to make the area of the minor-like reference markportion(s) that is included in the reference mark zone 351 as smallpossible, while also satisfying other reference mark designconsiderations, as described in greater detail below.

FIG. 4 is an isometric view of a fourth embodiment of a miniature fiberoptic readhead and scale arrangement 3000′ that includes a referencesignal generating configuration according to this invention. Theoperation of the miniature fiber optic readhead and scale arrangement3000′ is in many respects similar to that of miniature fiber opticreadhead and scale arrangement 3000 of FIG. 3, and similarly numberedcomponents may be similar or identical in form and operation, except asotherwise indicated below. Generally, only the significant differencesbetween the operation of the miniature fiber optic readhead and scalearrangements 3000′ and 3000 are described below.

As shown in FIG. 4, the miniature fiber optic readhead and scalearrangement 3000′ includes the integrated readhead 300 and a scalemember 81, which has a single scale track that includes a scale grating80 and a reference mark zone 451 and a reference mark boundary zone 451′located in an end region 89. The reference mark boundary zone 451′ mayinclude a track portion that is a grating portion (e.g., identical tothe scale grating 80, in various embodiments) to provide a firstboundary for the reference mark zone 451. The scale grating 80 may be aphase grating configured to suppress zero-order reflection, and providesa second boundary of the reference mark zone 451. It will be appreciatedthe boundary between the reference mark zone 451 and the scale grating80 may approximately correspond to the end of the incrementaldisplacement measuring range that is provided by the readhead and scalearrangement 3000′. Outside of zones 451 and 451′ the end region 89 maygenerally comprise a mirror-like region.

The reference mark zone 451 may include a minor-like reference markportion and because it is bounded along the measuring axis direction bythe scale grating 80 and the grating portion of the reference markboundary zone 451′, it may be substantially similar in structure andoperation to the reference mark zone 351 previously described withreference to the readhead and scale arrangement 3000. One advantage ofthe readhead and scale arrangement 3000′ over the readhead and scalearrangement 3000 is that the reference mark zone 451 is not locatedwithin the normal incremental displacement measuring range, where itmight disturb the accuracy of the normal incremental displacementmeasurements to some extent.

In one embodiment, the reference mark boundary zone 451′, being boundedby the minor portion of the reference mark zone 451 and the minor regionof the end portion 89, may be substantially similar or identical instructure to the reference mark zone 251′ previously described withreference to the readhead and scale arrangement 2000. In such anembodiment, the reference mark boundary zone 451′ may be used to providea secondary reference mark signal. In particular, the integratedreadhead 300 generally receives a significant amount of zero orderreflected light from the minor-like end region 89 and reference markportion(s) in the reference mark zone 451. However, when theillumination spot 353 overlaps the grating portion in the reference markboundary zone 451′ the zero order reflection is suppressed and asignificant portion of the reflected light is diffracted away from theintegrated readhead 300 as +/−first and third order diffracted light,according to previously described principles. As a result, the amount oflight that is received and transmitted as a reference signal by the endsof certain outer optical fibers 330 that have no spatially filtering,will be modulated as a function of the amount of overlap of theillumination spot 353 and the reference mark boundary zone 451′. Whenthe grating portion in the reference mark boundary zone 451′ has anoperational length LETOE, determined according to principles describedfurther below, a secondary reference mark location can be preciselydetermined, if desired.

In various embodiments, the signals from the reference mark zone 451 maybe used to determine the a reference mark location along the scalemember 81, and to signal the end of the incremental displacementmeasuring range of the readhead and scale arrangement 3000′. In someembodiments, signals from the reference mark boundary zone 451′ may beused to initiate a routine or circuit which acts as a “limit switch” forrelative displacement of readhead and scale arrangement 3000′, and/orthat prepares a circuit to detect the location of the reference markindicated by the reference mark zone 451 when an appropriate directionof relative displacement is provided.

FIG. 5 is an isometric view showing one exemplary embodiment of agrating and reference mark structure 500 according to this invention.Components numbered similarly to those shown in FIGS. 1-4 may be similaror identical. The grating and reference mark structure 500 comprises ascale grating 80 and a mirror portion reference mark 50-M located on ascale member 81. The minor portion reference mark 50-M is illustrated asa generic example. As shown in FIG. 5, the scale grating 80 includesgrating elements E extending along the direction of the Y-axis, that areseparated by recessed elements G. Grating elements E are arrangedperiodically along the measuring axis 82, according to a grating pitchP_(g). Each of the grating elements E has a width W_(E) along thedirection of the measuring axis 82, while each of the recessed elementsG has a width W_(G). The grating elements E also have a recess heightH_(E) along the direction of the Z-axis. The particular embodiment ofthe scale grating 80 shown in FIG. 5 is designed to suppress zero orderreflected light and all even diffraction orders. Methods foraccomplishing this are described in the incorporated '696 patent, andare otherwise known in the art. Briefly, in one exemplary embodiment,the scale grating 80 may be formed as a reflective phase grating with areflective chrome coating on both the rectangular grating elements E andthe recessed elements G, and with a recess height H_(E) between thegrating elements that causes destructive interference of the zero orderreflected light, for example a height of ¼ of the wavelength of thesource light used with the grating and reference mark structure 500. A50% duty cycle, that is, W_(E) approximately equal to W_(G), contributesto the best suppression of the 0^(th) order reflected light and alsosuppresses the rest of the even diffraction orders.

A mirror portion reference mark 50-M, having a length LETOE along theX-axis direction, may be located within the scale grating 80. Of coursethe scale grating 80 should remain in phase on each side of the minorportion reference mark 50-M. In various exemplary embodiments, the minorportion reference mark 50-M is sized and located such that itsboundaries are in phase with similar boundaries of the grating elementsE. In some embodiments, the length LETOE may coincide with (N+½) periodsof the scale grating 80, where N is an integer. In some embodiments, Nmay be chosen in the range of 10 to 30 periods of the scale grating 80.However, this range is exemplary only, and not limiting. Considerationsrelated to selecting a desirable length LETOE are described in greaterdetail below. It will be appreciated that the mirror portion referencemark 50-M is illustrated as being formed to correspond to the plane ofthe grating elements E, but it could alternatively be formed tocorrespond to the plane of the grating elements G. It should beappreciated that, in various embodiments, the mirror portion referencemark 50-M may be fabricated using a subset of the same steps used tofabricate the scale grating 80. Thus, the embodiment of the referencemark 50-M shown in FIG. 5 provides a particularly economical way ofproviding a reference mark that can be used in a miniature fiber opticreadhead and scale arrangement that includes a phase-type scale grating80 and an interferometric-type miniature fiber optic readhead. Asillustrated in FIG. 5, the grating and reference mark structure 500 isparticularly suitable for use in the scale track 88 of the readhead andscale arrangement 1000 shown in FIG. 1, where the mirror portionreference mark 50-M would be located in the reference mark zone 251, andin the readhead and scale arrangements 3000 and 3000′ shown in FIGS. 3and 4, where the minor portion reference mark 50-M would be located inthe reference mark zones 351 and 451, respectively.

It will be appreciated the roles of the grating and minor features shownin FIG. 5 and described above can be reversed to provide a gratingportion reference mark, having a length LETOE, embedded in a minor scaletrack portion extending along the X-axis direction, and/or in a mirrorregion such as that included in the end region 89 shown in FIG. 4. Itshould be appreciated that such a grating portion reference mark, and anassociated minor scale track portion or mirror region, may be fabricatedusing the same steps that are used to fabricate an incrementalmeasurement scale grating along an incremental measuring scale track ona scale member 81, thus providing a particularly economical way ofproviding a grating-type reference mark portion that can be used in aminiature fiber optic readhead and scale arrangement that includes aphase-type incremental measurement scale grating and aninterferometric-type miniature fiber optic readhead. Such a structureincluding a grating-type reference mark portion would be particularlysuitable for use in the scale track 88′ of the readhead and scalearrangement 2000 shown in FIG. 2, where the grating-type reference markportion would be located in the reference mark zone 251′, and in thereadhead and scale arrangements 3000′ shown in FIG. 4, wheregrating-type reference mark portion would be located in the referencemark zone 451′.

FIGS. 6A and 6B are isometric views schematically showing variousaspects of a first embodiment of a reference signal generatingconfiguration 6000 according to this invention, in relation to anillustrated dimension relationship. As illustrated in FIG. 6A, thereference signal generating configuration 6000 comprises a referencemark readhead optical fiber arrangement 600 operably positioned relativeto a generic reference mark 50A, which may comprise either a minorreference mark portion or a grating reference mark portion (e.g., on ascale member 81, not shown), depending on whichever is appropriate in aparticular readhead and scale arrangement, as previously described. Thereference mark 50A has a dimension LETOE along the X-axis or measuringaxis direction, a dimension WY along the Y-axis direction and a centerline RMC along the X-axis direction. Briefly, in operation, thereference mark readhead optical fiber arrangement 600 outputs adiverging source light 650 from a central fiber, which illuminates thereference mark 50A, as illustrated by exemplary source light rays 650-1,650-2, and 650-3. In various embodiments, the source light 650 isadvantageously monochromatic and spatially coherent, and may betemporally coherent in some embodiments. If the reference mark 50A is amirror reference mark portion surrounded by a grating track portionand/or scale track, then it will strongly reflect zero-order light backto the reference mark readhead optical fiber arrangement 600, asillustrated by the exemplary scale light rays 654-1, 654-2 and 654-3,which correspond to the source light rays 650-1, 650-2, and 650-3,respectively. In such a case, the reference mark signal effect region50A-SE corresponding to a mirror reference mark portion will be a regionof “signal increase”, to the extent that it overlaps any fiber opticreference mark signal receiver channel aperture provided by thereference mark readhead optical fiber arrangement 600.

As shown in FIG. 6A, the reflected zero order scale light produces areference mark signal effect region 50A-SE proximate to the opticalsignal receiver apertures provided by the ends of the optical fiberarrangement 600. The reference mark signal effect region 50A-SE has“magnified” dimensions that are twice those of the reference mark 50A,due to the divergence of the reflected scale light. In general, due tothe use of diverging source light, all reference mark signal effectregions described herein will have dimensions that are twice those oftheir corresponding reference marks, unless they are zone grating typereference marks. The reference mark signal effect region 50A-SE has acenter line RMC-SE along the along the X-axis direction. It will beappreciated that despite the size difference between the reference mark50A and the reference mark signal effect region 50A-SE, theircenterlines RMC and RMC-SE may be aligned along the measuring axis 82,and may displace at the same rate.

The previous description of operation has assumed that the referencemark 50A is a mirror reference mark portion (e.g., as shown in FIGS. 1and 3). If the reference mark 50A is a grating reference mark portionsurrounded by a mirror region or track portion (e.g., as shown in FIG.2), then the exemplary source light rays 650-1, 650-2, and 650-3 and thecorresponding reflected exemplary scale light rays 654-1, 654-2 and654-3 may be interpreted as zero-order light rays that are ordinarilyprovided by the surrounding minor region or track portion (e.g., theminor portion of the scale track 88′), but that are disrupted by thezero-order reflection suppression and higher order diffractionproperties of the grating portion reference mark 50A. In such a case,the corresponding reference mark signal effect region 50A-SE will be aregion of “signal decrease”, to the extent that it overlaps any fiberoptic reference mark signal receiver channel aperture provided by thereference mark readhead optical fiber arrangement 600. It should beappreciated that a grating portion reference mark is not limited tohaving grating bars aligned and spaced identically to the scale grating80. More generally, any grating portion reference mark that diffracts asignificant amount of source light away from the reference mark readheadoptical fiber arrangement 600 and/or significantly suppresses zero orderreflection may be used (e.g., two-dimensional gratings, etc.).

FIG. 6B shows a portion 6000′ of the reference signal generatingconfiguration 6000 shown in FIG. 6A, including the reference markreadhead optical fiber arrangement 600 and the reference mark signaleffect region 50A-SE. As shown in FIG. 6B, the reference mark readheadoptical fiber arrangement 600 may comprise receiving fibers 690R1,690R1′, 690R2, and 690R2′, having ends that provide reference marksignal receiver channel apertures that receive and provide the opticalreference mark signals REF1, REF 1′, REF2 and REF2′, as shown. The othertwo fibers 690× and 690X′ may be optional dummy fibers, used tofacilitate a close-packing assembly technique, if desired.Alternatively, in some embodiments, they may provide receiver aperturesthat are used to receive optical signals that may be useful formonitoring optical signal power variations, contamination effects, orother anomalies, or to provide additional light sources. The receivingfibers 690R1, 690R1′, 690R2, and 690R2′ may be multi-mode fibers havingan outer diameter DRF and a light carrying core area having a diameterDRA that may coincide with and/or provide a reference mark signalreceiver channel aperture in some embodiments. A central source fiber670 provides a light source 680, which generally emits a divergingsource light, and may be provided by the end of a single-mode core ofthe source fiber 670, in some embodiments.

In various embodiments, it may be advantageous to configure a readheadoptical fiber arrangement such that all optical fibers located within afiber optic readhead are located within a cylindrical volume having adiameter of at most 1.5 millimeters, or 1.0 millimeters, or less. In oneexemplary embodiment of the reference mark readhead optical fiberarrangement 600, the light carrying core diameter DRA may beapproximately 200 microns, which may also be the reference mark signalreceiver channel aperture diameter, the outer diameters DRF may beapproximately 250 microns, and the central fiber 670 may have the sameouter diameter DRF and a single-mode core diameter of approximately 4-10microns. Accordingly, in such an embodiment, the reference mark readheadoptical fiber arrangement 600 may have an overall diameter on the orderof 750 microns. However, it will be appreciated that in otherembodiments, larger or smaller fibers and/or other fiber spacings may beused. In FIG. 6B, dashed lines AR1L, AR1R, and CAR1 show the positionsof the left and right boundaries and center location, respectively, ofthe reference mark signal receiver channel apertures corresponding tothe signals REF1 and REF1′, along the X-axis direction. Dashed linesAR2L, AR2R, and CAR2 show the positions of the left and right boundariesand center location, respectively, of the reference mark signal receiverchannel apertures corresponding to the signals REF2 and REF2′ along theX-axis direction. The dimension LCAR1CAR2 denotes the distance along theX-axis between the effective centers of the reference mark signalreceiver channel apertures corresponding to signals REF1 and REF1′ andthe centers of the reference mark signal receiver channel aperturescorresponding to signals REF2 and REF2′. The dimension AR12SEP denotesthe separation distance between the boundaries AR1R and AR2L. Moregenerally, as used here and in reference to FIGS. 10 and 11, thedimension AR12SEP denotes the distance along the measuring axisdirection between the interior boundaries of two reference mark signalreceiver channel apertures that are included in an optical fiberarrangement, that is, between their boundaries that are closest to oneanother along the measuring axis direction. The dimension AR12SPANdenotes the total distance spanned between the boundaries ARIL and AR2R.More generally, as used here and in reference to FIGS. 10 and 11, thedimension AR12SPAN denotes the distance spanned along the measuring axisdirection between the exterior boundaries of two reference mark signalreceiver channel apertures that are included in an optical fiberarrangement, that is, between their boundaries that are farthest fromone another along the measuring axis direction.

For the reference signal generating configuration 6000, the most generalguidelines are that the reference mark readhead optical fiberarrangement 600 and the reference mark 50A should be configured suchthat such that the following relationship is fulfilled

AR12SEP<(2*LETOE)<AR12SPAN  (Eq. 1)

and such that the resulting reference mark signals are usable to definea reference position with a desired accuracy and/or repeatability withina signal crossing region proximate to the reference mark 50A, asdescribed in greater detail below. In various embodiments, aconfiguration that furthermore fulfills the relationships

(2*LETOE)>[AR12SEP+(0.25*(AR12SPAN−AR12SEP))]

(2*LETOE)<[AR12SEP+(0.75*(AR12SPAN−AR12SEP))]  (Eqs. 2 & 3)

may be advantageous (e.g., by providing a more robust and/or reliablerelationship between the reference mark signals). In various otherembodiments, a configuration that furthermore fulfills the relationships

(2*LETOE)>[AR12SEP+(0.4*(AR12SPAN−AR12SEP))]

(2*LETOE)<[AR12SEP+(0.6*(AR12SPAN−AR12SEP))]  (Eqs. 4 & 5)

may be more advantageous. In some embodiments, it may be mostadvantageous if the dimension 2*LETOE is approximately equal to[AR12SEP+(0.5*(AR12SPAN−AR12SEP))], or approximately equal to theeffective center to center distance LCAR1CAR2 between the reference marksignal receiver channel apertures corresponding to signals REF1 andREF2, to provide reference mark signals approximately as described belowwith reference to FIGS. 8 and 12.

FIG. 7 is an isometric view of a portion 7000′ of a second embodiment ofa reference signal generating configuration according to this invention.The design and operation of the portion 7000′ is in many respectssimilar to that of the portion 6000′ of FIG. 6B, and similarly numberedelements in the 6XX and 7XX series of numbers (e.g., the elements 690R2and 790R2) may be similar or identical in form and operation, except asotherwise indicated below. Generally, the design and operation of theportion 7000′ may be understood based on the previous description of theportion 6000′and the reference signal generating configuration 6000.Therefore, only the significant differences between the operation of theportions 6000′ and 7000′ are described below.

The primary difference between the reference mark readhead optical fiberarrangements 700 and 600 is that the optical fiber arrangement 700 has adifferent rotational orientation in the XY plane, allowing the ends offibers which are adjacent to one another along the X-axis direction toprovide the reference mark signal receiver channel apertures thatreceive and provide the optical signals REF1, REF1′, REF2 and REF2′(corresponding to the fibers 790R1, 790R1′, 790R2, and 790R2′,respectively). A reference signal generating configuration correspondingto the portion 7000′ may generally be configured according to thedimensional considerations and signal considerations outlined above withreference to EQUATIONS 1-5. In some embodiments, it may be mostadvantageous if the dimension 2*LETOE is approximately equal to[AR12SEP+(0.5*(AR12SPAN-AR12SEP))], or approximately equal to theeffective center to center distance LCAR1CAR2 between the reference marksignal receiver channel apertures corresponding to signals REF1 andREF2, to provide reference mark signals approximately as described belowwith reference to FIG. 8. It may be noted that since the dimensionLCAR1CAR2 is less for the optical fiber arrangement 700 than for theoptical fiber arrangement 600, the dimension 2*LETOE of the referencemark signal effect region 50B-SE and corresponding dimension LETOE ofthe corresponding reference mark (hereby designated as a reference mark50B, not shown) are selected to be less than for the reference signalgenerating configuration corresponding to the portion 7000′.

As illustrated in FIGS. 6A, 6B and 7, either the reference signalgenerating configuration 6000, or a reference signal generatingconfiguration corresponding to the portion 7000′, is suitable for use inthe readhead 200 and scale track 88 of the readhead and scalearrangement 1000 shown in FIG. 1, where a mirror portion reference markwould be located in the reference mark zone 251. Either configuration isalso suitable for use in the readhead 200 and scale track 88′ of thereadhead the readhead and scale arrangement 2000 shown in FIG. 2, wherea grating portion reference mark would be located in the reference markzone 251′.

FIG. 8 is a diagram showing two schematic signal charts 60 and 70, whichrespectively correspond to the reference signal generating configuration6000 of FIGS. 6A and 6B, and a reference signal generating configurationcorresponding to the portion 7000′ shown in FIG. 7. The signal chart 60,corresponding to the reference signal generating configuration 6000 ofFIGS. 6A and 6B, shows two reference signals, a combined signal(REF1+REF1′) and a combined signal (REF2+REF2), as a function ofrelative position along the measuring axis 82 between the reference marksignal effect region 50A-SE (or the reference mark 50A), and thereference mark readhead optical fiber arrangement 600. In particular,the point 61 corresponds to a position where the center line RMC of thereference mark 50A coincides with a position displaced by the lengthLETOE to the left of the position ARIL shown in FIG. 6B. Accordingly,the reference mark signal effect region 50A-SE does not overlap anyreference mark signal receiver channel apertures and no significantsignal is produced at the point 61. As the reference mark 50A isdisplaced to the right, the reference mark signal effect region 50A-SEincreasingly overlaps the REF1 and REF1′ reference mark signal receiverchannel apertures until a maximum is reached at a point 62, after adisplacement equal to the reference mark signal receiver channelaperture diameter (e.g., the light carrying core diameter DRA). As thereference mark 50A continues to displace to the right, no further signalchange is observed until the points 63 and 63′, which mark the leftlimit of a signal crossing region where the (REF1+REF1′) signal and the(REF2+REF2′) signal converge (or diverge) to (or from) a common value.In the signal crossing region, as the center line RMC of the referencemark 50A is displaced to the right of the position (AR1L+LETOE), thesignal (REF1+REF1′) begins to decrease as the overlap between thereference mark signal effect region 50A-SE and the REF1 and REF1′reference mark signal receiver channel apertures decreases. Because thereference mark signal receiver channel apertures have similar dimensionsand the length LETOE of the reference mark 50A is selected such that thedimension 2*LETOE of the reference mark signal effect region 50A-SE isapproximately equivalent to the dimension LCAR1CAR2 shown in shown inFIG. 6B, the signal (REF2+REF2′) simultaneously begins to increase atthe point 63′, as the overlap between the reference mark signal effectregion 50A-SE and the REF2 and REF2′ reference mark signal receiverchannel apertures increases. At a point 64, the center line RMC of thereference mark 50A is symmetrically located between the REF1/REF1′reference mark signal receiver channel apertures and the REF2/REF2′reference mark signal receiver channel apertures (at the position shownin FIG. 6B) and the signals (REF1+REF1′) and (REF2+REF2′) are thereforenominally equal. The behavior of the signals (REF1+REF1′) and(REF2+REF2′) at the remaining points 65, 65′, 66 and 67 may beunderstood by analogy with the previous explanation. The points 65 and65′, analogous to the points 63 and 63′, mark the right limit of thesignal crossing region.

The signal chart 70, corresponding to a reference signal generatingconfiguration that includes and corresponds to the portion 7000′ shownin FIG. 7, is analogous to the signal chart 60, described above. Thatis, the point 71 is analogous to the point 61, and so on. Thus, thebehavior of the signals (REF1+REF1′) and (REF2+REF2′) at the points71-77 may be understood by analogy with the previous explanation, incombination with the description of FIG. 7. In particular, in the signalcrossing region, as the center line RMC of the reference mark 50B isdisplaced to the right of the position (AR1L+LETOE), at the point 73 thesignal (REF1+REF1′) begins to decrease as the overlap between thereference mark signal effect region 50B-SE and the REF1 and REF1′reference mark signal receiver channel apertures decreases. Because thereference mark signal receiver channel apertures have similar dimensionsand the length LETOE of the reference mark 50B is selected such that thedimension 2*LETOE of the reference mark signal effect region 50B-SE isapproximately equivalent to the dimension LCAR1CAR2 shown in FIG. 7, thesignal (REF2+REF2′) simultaneously begins to increase at the point 73′,as the overlap between the reference mark signal effect region 50 b-SEand the REF2 and REF2′ reference mark signal receiver channel aperturesincreases. At a point 74, the center line RMC of the reference mark 50Bis symmetrically located between the REF1/REF1′ reference mark signalreceiver channel apertures and the REF2/REF2′ reference mark signalreceiver channel apertures (at the position shown in FIG. 7) and thesignals (REF1+REF1′) and (REF2+REF2′) are therefore nominally equal. Itwill appreciated that the features of the signals (REF1+REF1′) and(REF2+REF2′) of the signal chart 70 are spaced more closely along themeasuring axis than those of the signal chart 60, because the referencemark length LETOE is shorter in the reference signal generatingconfiguration corresponding to the signal chart 70, and thecenter-to-center spacing between the REF1/REF1′ reference mark signalreceiver channel apertures and the REF2/REF2′ reference mark signalreceiver channel apertures is also less.

In various exemplary embodiments, in order to provide a referenceposition along the measuring axis 82 in a robust manner, a referenceposition detection circuit may identify the position where theindividual signals (REF1+REF1′) and (REF2+REF2′) cross and are equal asthe reference position. It will appreciated based on the abovedescription that in various embodiments that use either a gratingportion or a minor portion as a reference mark, selecting theedge-to-edge length LETOE according to the dimensional considerationsand signal considerations outlined above in relation to EQUATIONS 1-5generally provides a reference signal generating configuration thatprovides reference mark signals that are adequate to define a referenceposition (e.g., where two respective reference signals have equalvalues) within a signal crossing region proximate to a reference mark.Fulfilling the relationships of EQUATIONS 2 and 3, or 4 and 5, mayprovide a particularly reliable and/or robust relationship between thereference mark signals in the signal crossing region. The referencesignal generating configurations corresponding to the signal charts 60and 70 each fulfill these relationships, and thereby insure a robustsignal crossing region that includes signals that nominally cross at asignal value approximately halfway between their maximum and minimumvalues, as shown in FIG. 8. It will be appreciated that the signalpolarities shown in FIG. 8 may generally correspond to those produce bya mirror portion reference mark surrounded by a grating region. Allsignals would generally be inverted for a grating portion reference marksurrounded by a minor region. In practice, all the signals willgenerally include common mode DC offsets, which are not shown in FIG. 8.In any case, reference signal generating configurations such as thosedisclosed above and further below, designed in accordance with thedimensional considerations and signal considerations outlined above withreference to EQUATIONS 1-5, can provide a plurality of respectivereference mark signals that define a reference position that isrepeatable to within less than one-half period of desirable spatiallyperiodic incremental measurement signals within the signal crossingregion proximate to a reference mark, such that the reference mark canreliably indicate a particular period or cycle of the incrementalmeasuring signals and the associated particular wavelength along thescale. For example, repeatability within less than 4, or 2, microns isreadily achieved, and submicron repeatability may be achieved,especially when the relationships of EQUATIONS 2 and 3, or 4 and 5, arefulfilled.

FIGS. 9 and 10 are isometric views schematically showing various aspectsof the operation of an integrated reference signal and incrementalsignal generating configuration 8000 (also referred to as the integratedsignal generating configuration 8000) according to this invention,including a third embodiment of a reference signal generatingconfiguration according to this invention. The view shown in FIG. 9 doesnot show a phase mask element 861, which is an essential element of theintegrated signal generating configuration 8000, in order to moreclearly illustrate other aspects of its operation. The phase maskelement 861 is described below with reference to FIG. 10.

FIG. 9 shows certain elements of the integrated signal generatingconfiguration 8000, including an integrated readhead optical fiberarrangement 800 (less the phase mask element 861, shown in FIG. 10)operably positioned relative to a generic minor portion reference mark50C surrounded by the scale grating 80 (e.g., on a scale member 81, notshown). The reference mark 50C has a dimension LETOE along the X-axisdirection, and a centerline RMC, as shown. Briefly, in operation, theintegrated readhead optical fiber arrangement 800 outputs a divergingsource light 850 from a light source 880 provided by a central fiber,which illuminates the reference mark 50C and the surrounding scalegrating 80 at the illumination spot 853. In various embodiments, thesource light 850 is advantageously monochromatic and spatially coherent,and may be temporally coherent in some embodiments. The scale grating 80provides reflectively diffracted +/−first order scale light 855A and855B, represented by its central rays in FIG. 9. The +/−first-orderscale lights 855A and 855B illuminate regions 855A′ and 855B′,respectively, which overlap in an interference zone 856 to forminterference fringes 866 proximate to a receiver plane 860 of theintegrated readhead optical fiber arrangement 800. The interferencefringes are spatially filtered by the phase mask element 861 (shown inFIG. 10) over the ends of the optical fibers that receive the opticalquadrature signals A, A−, B and B−, according to previously describedprinciples, and as described in greater detail below with reference toFIG. 10.

Simultaneously with the quadrature signal generating operation outlinedabove, when the minor portion reference mark 50C falls within theillumination spot 853, it reflects the diverging coherent source light850 to provide the reference mark signal effect region 50C-SE, which hasdimensions that are twice those of the mirror portion reference mark 50Cand a centerline RMC-SE that is aligned with the centerline RMC,according to principles previously outlined with reference to FIG. 6A.Additional details regarding the operation of the integrated referencesignal and incremental signal generating configuration 8000 aredescribed below with reference to FIG. 10.

FIG. 10 shows a portion 8000′ of the integrated signal generatingconfiguration 8000 shown in FIG. 9, including the integrated readheadoptical fiber arrangement 800, the reference mark signal effect region50C-SE, and a phase mask element 861. In order to more clearlyillustrate the phase mask element 861, FIG. 10 does not show theinterference fringes 866 in the interference zone 856, but it should beunderstood that such fringes are present during operation, as previouslydescribed. As shown in FIG. 10, the integrated readhead optical fiberarrangement 800 may comprise a central source fiber 870 that providesthe light source 880, which may be provided by the end of a single-modecore of the source fiber 870 in some embodiments, and receiver fibers890A, 890A′, 890B, 890B′, 890R1, and 890R2, that provide receiverchannels that receive the optical signals A, −A, B, −B, REF1, and REF2,respectively, as shown. The integrated readhead optical fiberarrangement 800 also includes a phase mask element 861 comprising phasemasks 820A, 820B, 820A′, 820B′, blocking masks 820BR1 and 820BR2, andopen aperture masks 820R1 and 820R2. Dashed lines ARIL, AR1R and CAR1show the positions of the left and right boundaries and effective centerlocation, respectively, of the reference mark signal receiver channelaperture corresponding to the signal REF1, and dashed lines AR2L, AR2Rand CAR2 show the positions of the left and right boundaries andeffective center location, respectively, of the reference mark signalreceiver channel aperture corresponding to the signal REF2. Thedimension LCAR1CAR2 denotes the distance along the X-axis between theeffective centers of the reference mark signal receiver channelapertures corresponding to signals REF1 and REF2. The dimension AR12SPANdenotes the total distance spanned between the boundaries ARIL and AR2R.As previously outlined with reference to FIG. 6B, generally herein thedimension AR12SEP denotes the distance along the measuring axisdirection between the interior boundaries of two reference mark signalreceiver channel apertures that are included in an optical fiberarrangement, that is, between their boundaries that are closest to oneanother along the measuring axis direction. For the portion 8000′, thedimension AR12SEP is between the boundaries AR1R and AR2L and is zero,so it is not labeled in FIG. 10, in order to avoid confusion. It willappreciated based on the above description that in various embodimentsof the integrated signal generating configuration 8000 that may useeither a grating portion or a mirror portion as a reference mark, thatselecting the edge-to-edge length LETOE according to the dimensionalconsiderations and signal considerations outlined above in relation toEQUATIONS 1-5 generally provides a robust reference signal generatingconfiguration, especially when the relationships of EQUATIONS 2 and 3,or 4 and 5, are fulfilled.

It will be appreciated that whereas the light receiving area ofpreviously described reference mark signal receiver channel apertureshas been defined solely by the light carrying core area at the end oftheir respective reference mark signal receiver channel optical fibers,in the integrated readhead optical fiber arrangement 800 the lightreceiving area of the reference mark signal receiver channel apertures890R1 and 890R2 is defined partially by the boundary of the lightcarrying core area at the end of their respective receiver channeloptical fibers and partially by their respective blocking/open aperturemasks 820BR1/820R1 and 820BR2/820R2. Of course, in various otherembodiments, analogous aperture masks could completely circumscribe anddefine the light receiving area of reference mark signal receiverchannel apertures, if desired. The respective blocking/open aperturemasks 820BR1/820R1 and 820BR2/820R2 include no structure that isspatially periodic along the measuring axis direction, so that anyinterference fringe light received by the reference mark signal receiverchannel apertures 890R1 and 890R2 will not create a significantspatially periodic signal component that disturbs the desired referencemark signals.

Briefly, in operation, the phase masks 820A, 820B, 820A′, and 820B′ arelocated at the receiving plane 860, and spatially filter theinterference fringes in the interference zone 856 to providequadrature-type periodic incremental measurement signals A, A′, B, andB′, respectively. In one embodiment, the phase masks 820A, 820B, 820A′,and 820W have relative spatial phases of 0, 90, 180 and 270 degrees,respectively. It will be appreciated that the relative positions of themeasurement signals A, A′, B, and B′, are illustrative only, and notlimiting. In general, the phase masks may be configured to provide anydesired arrangement for the measurement signals A, A′, B, and B′.Various operation and design principles that are relevant to the phasemask element 861, as well as alternative spatial phase arrangements, aredescribed in the incorporated references.

Blocking masks 820BR1 and 820BR2 and open aperture masks 820R1 and 820R2are located at the receiving plane 860 to mask the ends of fibers 890R1and 890R2 and provide reference signals REF1 and REF2. It will beappreciated that in the absence of the blocking masks 820BR1 and 820BR2the ends of the receiver fibers 890R1 and 890R2, which are collocatedalong the direction of the measuring axis 82, would respond to variouspositions of the reference mark signal effect region 50C-SE withidentical signals. In contrast, the arrangement of the masks 820BR1,820BR2, 820R1 and 820R2 provides reference mark signal receiver channelapertures that are offset along the direction of the measuring axis 82,to provide reference signals REF1 and REF2 that exhibit a desirablesignal crossing region, as described further below with reference toFIG. 12.

Regarding the signals A, A′, B, and B′, as previously indicated, thefringes arising from the scale grating 80 are present continuouslyduring the operation of the integrated signal generating configuration8000, in order to continuously generate these quadrature signals, asoutlined above. In general, the fringes may be weakened when the mirrorportion reference mark 50C is present in the illumination spot 853,detracting from the amount of diffracted +/−first order scale light 855Aand 855B. In addition, the phase masks 820A, 820B, 820A′, and 820B′ willadmit a portion of the zero order reflected light included in thereference mark signal effect region 50C-SE, when it overlaps theirlocations. As a result, the amplitudes and offsets of the quadraturesignals A, A′, B, and B′ will generally be affected by the referencemark 50C, which may detract from the resulting incremental displacementmeasurement accuracy. Therefore, in some embodiments, signal processing(e.g., adaptive amplitude “gain control” and/or DC offset compensationand/or phase compensation, or the like) may be applied to the quadraturesignals A, A′, B, and B′ to at least partially negate such effects,and/or the length LETOE and/or area of the reference mark 50C may belimited to limit its disruptive effects.

Regarding the reference signals REF1 and REF2, as previously indicated,the fringes arising from the scale grating 80 are present continuouslyduring the operation of the integrated signal generating configuration8000. However, the open aperture masks 820R1 and 820R2 provide nospatial filtering over the ends of receiver fibers 890R1, and 890R2, andthe light from a plurality of fringes simply provides a relativelyconstant average amount of light to the reference signals REF1 and REF2,independent of displacement. In contrast, when the reference mark signaleffect region 50C-SE overlaps the locations of the open aperture masks820R1 and 820R2, its zero order reflected light significantly increasesthe reference signals REF1 and REF2, as a function of the amount ofoverlap.

In various embodiments, it may be advantageous to configure a readheadoptical fiber arrangement such that all optical fibers located within afiber optic readhead are located within a cylindrical volume having adiameter of at most 1.5 millimeters, or 1.0 millimeters, or less. In onespecific embodiment, the fibers 890 may have light carrying core areashaving diameters DRA of approximately 200 microns, outer diameters DRFof approximately 250 microns and the central fiber 870 may have the sameouter diameter DRF, and a single-mode core diameter, or mode-fielddiameter, of approximately 4-10 microns. Accordingly, in such anembodiment, the reference mark readhead optical fiber arrangement 800may have an overall diameter on the order of 750 microns. However, itwill be appreciated that in other embodiments, larger or smaller fibersand/or other fiber spacings may be used.

The scale grating 80 may have a grating pitch P_(g) of approximately 4microns, and the fringes 866 may have a similar pitch. The referencemark signal receiver channel apertures defined by the reference marksignal receiver fiber ends and the blocking/open aperture masks820BR1/820R1 and 820BR2820R2 may have a dimension on the order of 100microns along the direction of the measuring axis 82. The reference mark50C may have a length LETOE that is advantageously of approximately 38microns in one embodiment, which provides a desirable tradeoff betweenadequate reference mark signal strength and minimal disturbance to theincremental measurement signals. However, it should be appreciated thatthe dimensional relationships outlined above for LETOE are exemplaryonly, and not limiting. In various applications, additional designconsiderations may favor smaller or larger dimensions for LETOE.

FIG. 11 is an isometric view showing a portion 9000′ of a secondintegrated signal generating configuration according to this invention,including a fourth embodiment of a reference signal generatingconfiguration according to this invention. The design and operation ofthe portion 9000′ is in many respects similar to that of the portion8000′ of FIG. 10, and similarly numbered elements in the 8XX and 9XXseries of numbers (e.g., the elements 820R2 and 920R2) may be similar oridentical in form and operation, except as otherwise indicated below.Generally, the design and operation of the portion 9000′ may beunderstood based on the previous description of the portion 8000′ andthe integrated signal generating configuration 8000. Therefore, only thesignificant differences between the operation of the portions 8000′ and9000′ are described below.

FIG. 11 shows the portion 9000′ including the integrated readheadoptical fiber arrangement 900, the reference mark signal effect region50D-SE, and a phase mask element 961. The primary difference between theportions 8000′ and 9000′ is that the optical fiber arrangements 800 and900 have a different rotational orientation in the XY plane. In theoptical fiber arrangement 900, receiver fibers 990R1 and 990R2, whichare separated along the X-axis direction, provide the reference signalsREF1 and REF2. In addition, the reference mark signal effect region50D-SE includes two signal effect sub-regions 50D1-SE and 50D2-SE,having individual dimensions 2LSEG, and providing an interioredge-to-edge dimension 2*LETOE. It will be appreciated that acorresponding mirror portion reference mark, hereby designated 50D, (notshown) including two mirror sub-portions or reference mark portionsdesignated 50D1 and 50D2 having individual dimensions LSEG, andproviding an interior edge-to-edge dimension LETOE, provides the twoseparated signal effect sub-regions or reference mark portions 50D1-SEand 50D2-SE according to previously outlined principles. Although in theparticular embodiment shown in FIG. 11, the dimension 2*LETOEcorresponds to the distance between the interior boundaries of the twosignal effect sub-regions 50D1-SE and 50D2-SE, it should be appreciatedthat in an alternative embodiment, the relationship between thesub-regions 50D1-SE and 50D2-SE may be such that the distance betweentheir exterior boundaries (rather than their interior boundaries)corresponds to the same dimension 2*LETOE. In either case, it should beappreciated that by structuring a reference mark to include tworeference mark sub-portions that are separated, a total area of thereference mark is advantageously limited in comparison to a singleportion reference mark of similar overall length and its disruptiveeffects on periodic incremental measurement signals are thereforeadvantageously limited. In some embodiments, to provide a desirabletradeoff between adequate reference mark signal strength and minimaldisturbance to the incremental measurement signals, each of the tworeference mark portions may have a dimension along the measuring axisdirection which is at least 0.25*(AR12SPAN−AR12SEP) and at most0.75*(AR12SPAN−AR12SEP). In various other embodiments, each of the tworeference mark portions may have a dimension along the measuring axisdirection which is at least 0.4*(AR12SPAN−AR12SEP) and at most0.6*(AR12SPAN−AR12SEP).

In any of these embodiments, selecting the edge-to-edge length LETOEaccording to the dimensional considerations and signal considerationsoutlined above in relation to EQUATIONS 1-5 generally provides a robustreference signal generating configuration, especially when therelationships of EQUATIONS 2 and 3, or 4 and 5, are fulfilled. Thereference signals REF1 and REF2 provided by the integrated signalgenerating configuration corresponding to the portion 9000′ exhibit adesirable signal relationship in a signal crossing region, as describedfurther below with reference to FIG. 12.

As illustrated in FIGS. 9, 10 and 11, either the reference signalgenerating configuration 8000, or a reference signal generatingconfiguration corresponding to the portion 9000′, is suitable for use inthe readhead 300 of the readhead the readhead and scale arrangements3000 and 3000′ shown in FIGS. 3 and 4.

FIG. 12 is a diagram showing two schematic signal charts 810 and 910,which respectively correspond to the integrated signal generatingconfiguration 8000 shown in of FIGS. 9 and 10, and an integrated signalgenerating configuration corresponding to the portion 9000′ shown inFIG. 11. The signal charts 810 and 910 do not have the same vertical orhorizontal scaling and, in practice, all the signals will generallyinclude common mode DC offsets, which are not shown in FIG. 12.

The signal chart 810, corresponding to the reference signal generatingconfiguration 8000 of FIGS. 9 and 10, shows two reference signals, asignal REF1 and a signal REF2 as a function of relative position alongthe measuring axis 82 between the reference mark signal effect region50C-SE (or the reference mark 50C), and the reference mark readheadoptical fiber arrangement 800. In particular, the point 811 correspondsto a position where the center line RMC-SE of the reference mark 50Ccoincides with a position displaced by the length LETOE of the referencemark 50C to the left of the position ARIL shown in FIG. 10. Accordingly,the reference mark signal effect region 50C-SE does not overlap the REF1reference mark signal receiver channel aperture, and no significantsignal is produced at the point 811. As the reference mark 50C isdisplaced to the right, the reference mark signal effect region 50C-SEincreasingly overlaps the REF1 reference mark signal receiver channelaperture until a maximum is reached at a point 812, at a position wherethe centerline RMC-SE is a distance LETOE to the left of the positionAR1R shown in FIG. 10. In the signal crossing region, as the referencemark 50C continues to displace to the right from the point 812, thesignal REF1 begins to decrease as the overlap between the reference marksignal effect region 50C-SE and the REF1 reference mark signal receiverchannel aperture decreases. The signal REF2 simultaneously begins toincrease at the point 812′, as the overlap between the reference marksignal effect region 50C-SE and the REF2 reference mark signal receiverchannel aperture increases. At a point 813, the center line RMC-SE ofthe reference mark 50C is symmetrically located along the REF1 referencemark signal receiver channel aperture and the REF2 reference mark signalreceiver channel aperture (at the position shown in FIG. 10) and thesignals REF1 and REF2 are therefore nominally equal. Because thereference mark signal receiver channel apertures have similar dimensionsand the length LETOE of the reference mark 50C is selected such that thedimension 2*LETOE of the reference mark signal effect region 50C-SE isapproximately equivalent to the dimension LCAR1CAR2 shown in shown inFIG. 10, the signals REF1 and REF2 cross at a value that isapproximately midway between their maximum and minimum values in thesignal crossing region, which leaves a robust margin for individualsignal variations that may arise from various causes. The behavior ofthe signals REF1 and REF2 at the remaining points 814, 814′ and 815 maybe understood by analogy with the previous explanation.

The signal chart 910 corresponds to a reference signal generatingconfiguration that includes and corresponds to the portion 9000′ shownin FIG. 11. The signal chart 810 shows two reference signals, a signalREF1 and a signal REF2 as a function of relative position along themeasuring axis 82 between the centerline RMC-SE-50D2 of the referencemark signal effect sub-region 50D2-SE and the optical fiber arrangement900. It will be appreciated that the centerline RMC-SE-50D2 coincideswith centerline of a corresponding reference mark sub-portion, herebydesignated as sub-portion 50D2 of a two-portion reference mark 50D,which also includes a sub-portion designated 50D1. The point 911corresponds to a position where the center line RMC-SE-50D2 coincideswith a position displaced by the length LSEG to the left of the positionAR2L shown in FIG. 11. Accordingly, the reference mark signal effectregion 50D2-SE does not overlap with the REF2 reference mark signalreceiver channel aperture, and no significant signal is produced at thepoint 811. As the reference mark 50D is displaced to the right, thereference mark signal effect region 50D2-SE increasingly overlaps theREF2 reference mark signal receiver channel aperture until a maximum isreached at a point 912, where the centerline RMC-SE-50D2 is a distanceLSEG to the left of the position AR2R shown in FIG. 11. In the signalcrossing region, as the reference mark 50D continues to displace to theright from the point 912, the signal REF2 begins to decrease as theoverlap between the reference mark signal effect region 50D2-SE and theREF2 reference mark signal receiver channel aperture decreases and thesignal REF1 simultaneously begins to increase at the point 912′, as theoverlap between the reference mark signal effect region 50D1-SE and theREF1 reference mark signal receiver channel aperture increases. At apoint 913, the reference mark 50D is symmetrically located between theREF1 and REF2 reference mark signal receiver channel apertures (at theposition shown in FIG. 11) and the signals REF1 and REF2 are thereforenominally equal. Because the reference mark signal receiver channelapertures have similar dimensions and the length LETOE of the referencemark 50D is selected such that the dimension 2*LETOE of the referencemark signal effect region 50D-SE is approximately equivalent to thedimension LCAR1CAR2 shown in shown in FIG. 11, the signals REF1 and REF2cross at a value that is approximately midway between their maximum andminimum values in the signal crossing region, which leaves a robustmargin for individual signal variations that may arise from variouscauses. The behavior of the signals REF1 and REF2 at the remainingpoints 914, 914′ and 915 may be understood by analogy with the previousexplanation.

The points 916-918 illustrate a secondary REF2 signal that is providedby the sub-region 50D1-SE overlapping the REF2 receiver, as thereference mark 50D continues to displace to the right. However, it willbe appreciated that there is no complementary “crossing signal” providedby the REF1 reference mark signal receiver channel aperturecorresponding to these points. An analogous REF1 signal, occurring for adisplacement to the left of the illustrated signal region, is not shown.Since the reference position is established where the REF1 and REF2signals are equal, in the signal crossing region, the secondary REF2signal corresponding to the points 916-918 is irrelevant, as is theanalogous secondary REF1 signal, except for their potential use asindicators that the reference position is approaching, or a confirmationthat the reference position should have been detected and is receding,depending on the displacement direction.

Higher Resolution Reference Mark Signal Generating Configurations

The reference mark signal generating configurations outlined abovegenerally provide a single reference mark signal having a first level ofresolution and repeatability. In various embodiments the first level ofresolution and repeatability may be on the order of 0.2 microns.Hereafter, the term “reference mark primary signal” may be used to referto reference mark signals generated as outlined above.

In some applications it may be desirable to provide reference marksignals offering an improved level of reference mark position resolutionand repeatability, in comparison to the reference mark primary signalsoutlined above. Hereafter, the term “reference mark secondary signals”may be used to refer to such reference mark signals that offer animproved level of resolution and repeatability. The reference signalgenerating configurations outlined below provide such reference marksecondary signals. Briefly, the embodiments outlined below withreference to FIGS. 13-18 are generally designed in accordance withEQUATION 1 (and/or EQUATION 2, 3, 4 or 5) and use principles outlinedabove with reference to FIGS. 1-12 to generate reference mark primarysignals, in combination with additional features outlined below, whichgenerate reference mark secondary signals. The reference mark secondarysignals indicate a reference mark position with higher resolution andrepeatability than the reference mark primary signals (e.g., resolutionand/or repeatability approximately ten times better, which may be on theorder of 20 nanometers). However, the reference mark secondary signalsare periodic. Therefore, a primary reference mark position or primarysignal crossing point derived from the reference mark primary signals isused to eliminate any potential secondary reference mark positionambiguity associated with that periodicity, as described in greaterdetail below.

In the previous embodiments that generate only reference mark primarysignals, the reference mark may comprise either a mirror-like referencemark portion located on a scale track that includes a grating (e.g., seereference scale track 88 in FIG. 1), or, alternatively, a grating typereference mark portion located on a mirror-like reference scale track(e.g., see reference scale track 88′ in FIG. 2). However, in theembodiments outlined below, which also generate reference mark secondarysignals, the reference mark cannot comprise a grating type referencemark portion surrounded by a minor-like track portion, for reasonsdescribed below.

It should be appreciated that in the following description, severalfigures employ a reference numbering convention that relates certainelements to corresponding or analogous elements in figures describedabove, such that they may be understood by analogy, without additionaldescription. According to this numbering convention, reference numbersthat have similar suffixes and/or form may be analogous and may havesimilar design and principles of operation (e.g., 1680 is analogous to880, 50G-SE is analogous to 50C-SE, etc.). This is similar to anumbering convention used frequently in FIGS. 1-12. However, inaddition, for elements that are related to generating a reference markprimary signal in the figures described below, a “P” may also beinserted into the suffix. Thus, for example, an element numbered1620BPR2 may be analogous to an element numbered 820BR2, based on thesimilar numerical suffix “20BR2” as modified with the inclusion of “P”for “primary”. Similarly, an element numbered 1390PR1 may be analogousto the element number 790R1, etc. In addition, for elements that arerelated to generating a reference mark secondary signal in the figuresdescribed below, an “S” may be inserted into a suffix used for ananalogous element. For example, except for the fact that it is used toprovide a reference mark secondary signal, a receiver fiber numbered1590SR1 may be analogous to a receiver fiber numbered 890R1, based onthe similar numerical suffix “90R1” as modified with the inclusion of“S” for “secondary”.

FIG. 13 is an isometric view schematically showing various aspects ofthe operation of a portion of a fifth embodiment of a reference signalgenerating configuration 13000 according to this invention. Thereference signal generating configuration 13000 generates reference markprimary signals according to principles previously outlined withreference to FIG. 7, and elements numbered with analogous numbers inFIGS. 13 and 7, may be similar or identical in form and operation,except as otherwise indicated below. FIG. 13 shows various elements ofthe reference signal generating configuration 13000, including areference mark readhead optical fiber arrangement 1300, the referencemark primary signal effect region 50E-SE, and a mask element 1361. Asshown in FIG. 13, the reference mark readhead optical fiber arrangement1300 may comprise receiving fibers 1390PR1, 1390PR1′, 1390PR2, 1390PR2′,which receive and provide reference mark primary signals PREF1, PREF1′,PREF2, PREF2′, and receiving fibers 1390SR1 and 1390SR2, which receiveand provide reference mark secondary signals SREF1 and SREF2 asdescribed further below.

The reference mark primary signal effect region 50E-SE may be analogousor identical to the reference mark signal effect region 50B-SE shown inFIG. 7. Furthermore, the receiving fibers 1390PR1, 1390PR1′, 1390PR2,1390PR2′, may be analogous or identical to the receiving fibers 790PR1,790PR1′, 790PR2, and 790PR2′ shown in FIG. 7. The operation of theseelements and the resulting signals may therefore be understood accordingto previously described principles.

The operation of the reference mark secondary signal receiver channels,comprising the receiving fibers 1390SR1 and 1390SR2 which carry thereference mark secondary signals SREF1 and SREF2, will now be outlined.As previously described (e.g., with reference to FIG. 9), when aminor-like reference mark portion is surrounded by grating trackportion, the grating track portion may provide reflectively diffracted+/−first order scale lights, which illuminate regions which overlap toform interference fringes in an interference zone proximate to areceiver plane of the mask element 1361. By analogy with previousfigures, FIG. 13 shows such an interference zone 1356-SSE, also referredto as the reference mark secondary signal effect region 1356-SSE. Itwill be understood that the interference fringes are locally disruptedor dominated by zero order reflected light in the vicinity of thereference mark primary signal effect region 50E-SE, in order to providethe reference mark primary signals PREF1, PREF1′, PREF2, PREF2′according to previously described principles.

The spatial filter masks 1320SR1 and 1320SR2, at the receiving plane ofthe mask element 1361, mask the receiver channel apertures provided bythe ends of the receiving fibers 1390SR1 and 1390SR2, and spatiallyfilter the interference fringes in the reference mark secondary signaleffect region 1356-SSE to provide the spatially periodic signals SREF1and SREF2, respectively. In various embodiments, the spatial filtermasks 1320SR1 and 1320SR2 have light-blocking elements arranged at apitch that is the same as the interference fringes, and are arrangedrelative to one another with a nominal spatial phase difference of 180degrees, to provide signals SREF1 and SREF2 as described in greaterdetail below with reference to FIG. 14. It will be appreciated that thelight blocking element pitch shown in FIG. 13, and following figures, isnot necessarily to scale and may be exaggerated for purposes ofillustration.

In order to provide reliable reference mark primary signals (e.g.,PREF1, PREF1′, PREF2, PREF2′), design considerations analogous to thosepreviously outlined with reference to FIGS. 6-11 may be applied (e.g.,by treating APR12SEP and APR12SPAN similarly to AR12SEP and AR12SPAN,etc.). In order to provide reliable reference mark secondary signals(e.g., SREF1, SREF2), additional design relationships are desirablebetween the dimensions and/or locations of the reference mark primarysignal effect region (e.g., the region 50E-SE) and the reference marksecondary signal receiver channel apertures (e.g., as provided by theends of the receiving fibers 1390SR1 and 1390SR2). To help explain theserelationships, in addition to dimensions analogous those previouslyoutlined with reference to FIGS. 6-11, FIG. 13 also shows dashed linesASR1L and ASR1R, and ASR2L and ASR2R, marking the left and rightboundaries, along the X-axis direction, of exemplary reference marksecondary signal receiver channel apertures provided by the receivingfibers 1390SR1 and 1390SR2.

A separation distance ASR12SEP is shown between ASR1R and ASR2L. Invarious embodiments according to this invention, it is beneficial toarrange a reference signal generating configuration (e.g., the referencesignal generating configuration 13000) such that:

ASR12SEP≧2*LETOE  (Eq. 6)

This relationship corresponds to a configuration wherein the referencemark primary signal effect region 50E-SE may be positioned between thereceiver channel apertures provided by the receiving fibers 1390SR1 and1390SR2, such that they derive desirable signals from the interferencefringe light, without being significantly affected by the zero orderreflected light of the primary signal effect region 50E-SE. At the sametime, the reference mark primary signal effect region 50E-SE may becentered relative to the receiver channel apertures provided by thereceiving fibers 1390PR1, 1390PR1′, 1390PR2, and 1390PR2′ in order togenerate desirable reference mark primary signals (e.g., PREF1, PREF1′,PREF2, PREF2′) in a desirable primary signal crossing region, allapproximately as shown in FIG. 13, and as described in greater detailbelow with reference to FIG. 14. In order to better insure suchdesirable signals, a clearance dimension PSCLR may be provided as shownin FIG. 13. PSCLR is the clearance from the edge of the reference markprimary signal effect region 50E-SE to the boundaries of reference marksignal secondary receiver channel apertures when the reference markprimary signal effect region 50E-SE is nominally centered between thoseboundaries along the measuring axis 82. Thus, in various embodiments:

ASR12SEP=(2*LETOE)+(2*PSCLR)  (Eq. 7)

In various embodiments it is desirable for PSCLR to be greater thanzero, or more desirably at least 10 microns, and in some embodiments atleast 25, and in other embodiments at least 50 microns.

FIG. 14 is a diagram including a signal chart 70′ schematically showingreference mark primary signals, and a signal chart 60′ schematicallyshowing reference mark secondary signals, all generated according tothis invention. In various embodiments, signal amplitudes, spatialperiods, etc., may vary from those shown in FIG. 14, which isillustrative only, and not limiting. For purposes of explanation, thesignals are described corresponding to the reference signal generatingconfiguration 13000 of FIG. 13. In particular, FIG. 14 shows a signalchart 70′ illustrating the combined reference mark primary signals(PREF1 +PREF1′) and (PREF2+PREF2′). Signal chart 70′ is analogous tosignal chart 70 of FIG. 8, with analogous dimension names adapted tocorrespond to the description of FIG. 13. Signal chart 70′ may thereforebe understood according to previously described principles, with thedifference that the primary reference position indicated by the signalcrossover point 74′ is regarded merely as a primary or first indicatorof the reference position. As illustrated by the dashed line extendingfrom the signal crossover point 74′ to the signal chart 60′, the signalcrossover point 74′ and/or the primary reference position has a specificfixed spatial relationship to the reference mark secondary signals SREF1and SREF2 shown in the signal chart 60′.

As shown in the signal chart 60′, and as previously explained, when areference mark readhead optical fiber arrangement according to thisinvention is moved along the measuring axis 82 relative to the referencemark on the scale, the reference mark secondary signals SREF1 and SREF2are spatially periodic with a period corresponding to the fringe pitch,and are 180 degrees out of phase with one another, due to thearrangement of the spatial filter masks 1320SR1 and 1320SR2. The signalchart 60′ also shows that when the reference mark primary signal effectregion 50E-SE crosses a receiver channel aperture corresponding to areference mark secondary signal, it may generally contribute a DC signalcomponent (e.g., the DC signal component SREF1DC or SREF2DC), that addsto the spatially periodic signal component corresponding to thatreceiver channel aperture. However, as shown in FIG. 14 and previouslyexplained, when a relationship according to EQUATION 6 and/or 7 isfulfilled and the reference mark primary signals are in the primarysignal crossing region, the reference mark secondary signals SREF1 andSREF2 need not include a significant DC signal component, which isadvantageous for reliably determining the high resolution secondaryreference position.

As schematically shown in FIG. 14, in general, a particular secondarysignal crossing region of the reference mark secondary signals SREF1 andSREF2 is significantly narrower than the primary signal crossing region.It will be appreciated that the signal relationships in FIG. 14 areshown schematically, for clarity of illustration. In practice, invarious embodiments, the secondary signal crossing region may be on theorder of 10 times narrower than the secondary signal crossing region.Thus, in practice, the position of a particular signal crossing point64′ of the secondary signals SREF1 and SREF2, and the correspondingsecondary reference position, may be determined with spatial resolutionand/or accuracy that is better (e.g., about 10 times better) than theprimary signal crossing point 74′ and/or primary reference position.

It will be appreciated that the particular signal crossing point 64′ maygenerally be indistinguishable from the other periodic signal crossingpoints that occur in its vicinity. However, the primary signal crossingpoint 74′ has a specific fixed spatial relationship to the referencemark secondary signals SREF1 and SREF2 shown in the signal chart 60′,and has a resolution and accuracy that is better +/−half a period of thesecondary signals SREF1 and SREF2. Therefore, the primary signalcrossing point 74′ may be used to reliably indicate or identify theparticular secondary signal crossing point 64′. Thus, a high resolutionsecondary reference position may be repeatedly and reliably determinedaccording to this invention, based on a particular secondary signalcrossing point 64′. One exemplary signal processing method fordetermining the high resolution secondary reference position is outlinedbelow with reference to FIG. 19.

FIG. 15 is an isometric view schematically showing various aspects ofthe operation of a portion of a sixth embodiment of a reference signalgenerating configuration 15000 according to this invention. Thereference signal generating configuration 15000 generates reference marksignals according to principles previously outlined with reference toFIG. 13, and may be understood by analogy with previous description.Therefore, only significant differences are described below. Elementsnumbered with analogous numbers in FIGS. 15 and 13 may be similar oridentical in form and operation, except as otherwise indicated below. Incomparison to the reference signal generating configuration 13000 shownin FIG. 13, the primary differences in the reference signal generatingconfiguration 15000 are that the dimension 2*LETOE of the reference markprimary signal effect region 50E-SE is significantly narrower than thatof the primary signal effect region 50E-SE shown in FIG. 13, and thatthe receiver channel apertures associated with the reference markprimary signals PREF1, PREF1′, PREF2, and PREF2′ are defined by the maskelement 1561. In particular, the mask element 1561 includes blockingmask portions 1520BPR1, 1520BPR1′, 1520BPR2 and 1520BPR2′ and openaperture mask portions 1520PR1, 1520PR1′, 1520PR2 and 1520PR2′, whichare positioned over the receiving fibers 1590PR1, 1590PR1′, 1590PR2 and1590PR2′, respectively. This arrangement provides circular (or othershapes, in various embodiments) receiver channel apertures that havedimensions and positions that are designed to complement a desireddimension 2*LETOE of the reference mark primary signal effect region50E-SE, according to previously described principles. In comparison tothe reference signal generating configuration 13000, the referencesignal generating configuration 15000 may provide a larger clearancedimension PSCLR, which is advantageous for reasons outlined previously.In some embodiments, the open aperture mask portions 1520PR1, 1520PR1′,1520PR2 and 1520PR2′ may be configured with a relative small dimensionalong the measuring axis 82, to provide a steeper reference mark primarysignal change in a narrower primary signal crossover region. In someembodiments, this may enhance the resolution and/or accuracy of theprimary signal crossing point 74′, which may help in reliablyidentifying a particular secondary signal crossover region and/or signalcrossover point. This may be particularly useful when the spatial periodof the reference mark secondary signals is small (e.g., on the order of4 microns). Otherwise, the design and operation of the reference signalgenerating configuration 15000 may be understood based the analogousdesign and operation previously described with reference to FIG. 13, andelsewhere herein.

FIG. 16 is an isometric view schematically showing various aspects ofthe operation of a portion of a seventh embodiment of a reference signalgenerating configuration 16000 according to this invention. It may beseen that in comparison to the embodiments shown in FIGS. 13 and 15,that the optical fiber arrangement 1600 shown in FIG. 16 has a differentrotational orientation in the XY plane. This allows the reference markprimary signal effect region 50E-SE to be positioned between the ends offour fibers, rather than two, to provide reference mark secondary signalreceiver channel apertures that receive and provide four secondarysignals SREF1, SREF1′, SREF2 and SREF2′ (corresponding to the fibers1690SR1, 1690SR1′, 1690SR2, and 1690SR2′, respectively). Otherwise, thedesign principles and operation of each of the receiver channelscorresponding to the secondary signals SREF1, SREF1′, SREF2 and SREF2′are analogous to those previously described with reference to FIG. 13.Elements numbered with analogous numbers in FIGS. 16 and 13 may havesimilar design principles and operation, and may be understood byanalogy unless otherwise indicated below. In the embodiment shown inFIG. 16, the spatial filter masks 1620SR1 and 1620SR1′ are arranged tohave the same spatial phase relative to the interference fringes in thereference mark secondary signal effect region 1356-SSE. Thus, thesecondary signals SREF1 and SREF1′ have the same spatial phase and maybe combined (e.g., added) during signal processing. The spatial filtermasks 1620SR1 and 1620SR1′ are also arranged to have the same spatialphase, which is approximately 180 degrees out of phase with the spatialfilter masks 1620SR1 and 1620SR1′. Thus, the secondary signals SREF2 andSREF2′ have the same spatial phase and may likewise be combined duringsignal processing. The combined secondary signals are analogous to theindividual secondary signals shown in signal chart 60′, and may besimilarly processed.

The reference signal generating configuration 16000 generates referencemark primary signals PREF1 and PREF2 using an optical fiber and maskarrangement similar or identical to that previously described withreference to FIG. 10 for generating the analogous signals REF1 and REF2.Elements numbered with analogous numbers in FIGS. 16 and 10, may havesimilar or identical design principles and operation. The design andoperation of the elements used to generate the reference mark primarysignals PREF1 and PREF2 may therefore be understood according topreviously described principles. The individual primary signals PREF1and PREF2, are analogous to the combined primary signals in signal chart70′ in FIG. 14, and may be similarly processed.

FIGS. 17A and 17B are illustrations 17000A and 17000B, respectively,showing alternative aperture mask configurations usable in place ofportions of the aperture mask configuration of the mask element 1661shown in FIG. 16. In particular, FIGS. 17A and 17B show alternativeaperture mask configurations for those portions of a mask elementassociated with the generating the reference mark primary signals PREF1and PREF2, which may also be referred to as primary apertureconfigurations. Elements numbered with analogous numbers in FIGS. 17A,17B and FIG. 16 may have similar design principles and operation, andmay be understood by analogy unless otherwise indicated below. It willbe understood that mask element portions not explicitly shown in FIGS.17A and 17B may be similar or identical to those illustrated for themask element 1661 in FIG. 16.

In comparison to the configuration of the blocking mask 1620BPR1, openaperture mask 1620PR1, blocking mask 1620BPR2 and open aperture mask1620PR2 shown in FIG. 16, the primary difference in FIG. 17A is that theblocking mask 1620BPR1′, open aperture mask 1620PR1′, blocking mask1620BPR2′ and open aperture mask 1620PR2′, provide left and rightaperture edges that extend transverse to the measuring axis direction,and those transverse aperture edges include zigzag portions that areangled relative to the Y-axis such that each transverse aperture edgespans a primary aperture edge transition dimension PAET extending froman adjacent aperture boundary (e.g., the boundary indicated by thedashed line APR2R) along the direction of the measuring axis 82 towardthe opposite edge of the aperture. A representative dimension PAET isshown only for the right edge of the open aperture mask 1620PR2′,however, it will be understood that analogous dimensions PAET exist foreach left and/or right aperture edge in FIG. 17A. The dimensions PAETneed not be the same for each aperture edge, although they may be, invarious embodiments. By angling the respective aperture edges to spantheir respective dimensions PAET, any interference fringes that passacross the open aperture masks 1620PR1′ and/or 1620PR2′ are spatiallyfiltered such that the amplitude of their disruptive periodic opticalsignal contribution to the reference mark primary signals is at leastpartially suppressed, particularly in the vicinity of the primary signalcrossing region described previously with reference to FIG. 14. It maybe advantageous, in various embodiments, if each dimension PAET is atleast one fringe pitch FP at the receiving plane of the mask element. Insome embodiments, each dimension PAET may be at least three times FP, ormore. In some embodiments, while not necessary, it may be advantageousif each dimension PAET is nominally equal to an integer number of fringepitches FP at the receiving plane of the mask element. It will beappreciated that the fringe pitch FP shown in FIGS. 17A and 17B is notnecessarily to scale and may be exaggerated for purposes ofillustration.

According to a separate aspect of the configuration shown if FIG. 17A,in some embodiments, while not necessary, it may be advantageous if aprimary aperture width dimension PAW is nominally equal to an integernumber of fringe pitches FP at the receiving plane of the mask element,at least along a majority of the length of the apertures edges along theY-axis direction. In such a case, the disruptive optical signalcontribution of any fringes that cross the open aperture masks 1620PR1′or 1620PR2′ will tend to be more constant, particularly in the vicinityof the primary signal crossing region, which is less disruptive to thereference mark primary signal crossing position than a variable signalcontribution from the fringes. The primary aperture width dimension PAWmay vary along the Y-axis direction or it may be constant. In any case,it may be defined at each location along the Y-axis, as the openaperture dimension along the direction of the measuring axis 82. Thisdesign feature may be used in combination with a non-zero primaryaperture edge transition dimension PAET, as shown in FIG. 17A, or it mayalso be beneficial in combination with straight aperture edges that arealigned along the Y-axis direction.

FIG. 17B illustrates a configuration which may be understood by analogyto the previous description of FIG. 17A. In particular, the blockingmask 1620BPR1″, open aperture mask 1620PR1″, blocking mask 1620BPR2″ andopen aperture mask 1620PR2″, provide left and right aperture edges thatextend transverse to the measuring axis direction and those transverseaperture edges are angled relative to the Y-axis such that eachtransverse aperture edge spans a primary aperture edge transitiondimension PAET extending from an adjacent aperture boundary (e.g., theboundary indicated by the dashed line APR2R) along the direction of themeasuring axis 82 toward the opposite edge of the aperture. Incomparison to the configuration of FIG. 17A, the primary difference isthat the apertures shown in FIG. 17B have the character of a “rotatedrectangle”, rather than having zigzag edges. Nevertheless, the aperturesshown in FIG. 17B have analogous dimensions PAET and PAW that mayfulfill the design principles previously outlined with reference forFIG. 17A. Based on the foregoing examples of FIGS. 17A and 17B, it willbe appreciated that apertures edges may alternatively have segments thatare curved (rather than straight and angled), or that meander in someother manner, over a primary aperture edge transition dimension PAET,and such designs may also fulfill the design principles previouslyoutlined with reference for FIG. 17A. Therefore, it will be appreciatedthat the foregoing examples are illustrative only, and not limiting. Itshould also be appreciated that apertures designed according to theprinciples outlined with reference to FIGS. 17A and 17B may generally beused in various other embodiments of the invention (e.g., in place ofthe reference signal apertures shown in FIG. 10 or 11, etc.).

FIG. 18 is an isometric view showing a portion of an eighth embodimentof a reference signal generating configuration 18000 according to thisinvention. The design and operation of the reference signal generatingconfiguration 18000 may be thought of as a two-channel reference markprimary signal generating configuration based generally on the referencemark signal generating teachings associated with FIG. 11, in combinationwith a four-channel reference mark secondary signal generatingconfiguration that is analogous to that outlined above with reference toFIG. 16. Thus, the design and operation of the reference signalgenerating configuration 18000 may generally be understood based onprevious description. Numbered elements in the 18XX series of numbersmay be similar or analogous in design principle, operation, and in somecases form, to similarly numbered elements in the 16XX series ofnumbers, except as otherwise indicated below. Therefore, onlysignificant differences between the reference signal generatingconfigurations 18000 and 16000 are described below.

The reference signal generating configuration 18000 includes theintegrated readhead optical fiber arrangement 1800, the reference marksignal effect region 50H-SE, and a phase mask element 1861. It should beappreciated that the reference mark signal effect region 50H-SE includestwo portions 50H1-SE and 50H2-SE. In general, the reference mark signaleffect region 50H-SE is analogous to the of the two-portion referencemark signal effect region 50D-SE shown in FIG. 11 and may be similarlyunderstood. Additional design considerations related to the referencemark signal effect region 50H-SE are outlined further below. It may beseen that in comparison to the embodiment shown in FIG. 16, that theoptical fiber arrangement 1800 shown in FIG. 18 has a differentrotational orientation in the XY plane, with the reference mark primarysignals PREF1 and PREF2 provided by the two fibers 1890PR1 and 1890PR2that are farthest from one another along the X-axis direction and thefour reference mark secondary signals SREF1, SREF1′, SREF2 and SREF2′provided by the four fibers 1890SR1, 1890SR1′, 1890SR2, and 1890SR2′that are closest to one another along the X-axis direction. Otherwise,the design principles and operation of each of the receiver channelscorresponding to the secondary signals SREF1, SREF1′, SREF2 and SREF2′are analogous to those previously described with reference to FIG. 16.In particular, the spatial filter masks 1820SR1 and 1820SR1′ arearranged to have the same spatial phase relative to the interferencefringes in the reference mark secondary signal effect region 1856-SSE.Thus, the secondary signals SREF1 and SREF1′ have the same spatial phaseand may be combined (e.g., added) during signal processing. The spatialfilter masks 1820SR1 and 1820SR1′ are also arranged to have the samespatial phase, which is approximately 180 degrees out of phase with thespatial filter masks 1820SR1 and 1820SR1′. Thus, the secondary signalsSREF2 and SREF2′ have the same spatial phase and may likewise becombined during signal processing. The combined secondary signals areanalogous to the individual secondary signals shown in signal chart 60′,and may be similarly processed.

The reference signal generating configuration 18000 generates referencemark primary signals PREF1 and PREF2 using an aperture maskconfiguration including the blocking mask 1820BPR1, open aperture mask1820PR1, blocking mask 1820BPR2 and open aperture mask 1820PR2, whichare configured according to design principles previously outlined withreference to FIG. 17A. The aperture mask configuration works incooperation with signal effect sub-regions 50H1-SE and 50H2-SE, whichhave individual dimensions 2LSEG, and an interior edge-to-edge dimension2*LETOE. It will be appreciated that the signal effect sub-regions areprovided by a corresponding mirror portion reference mark, herebydesignated 50H, (not shown) that includes two minor sub-portions orreference mark portions designated 50H1 and 50H2 having individualdimensions LSEG, and providing an interior edge-to-edge dimension LETOE.Although in the particular embodiment shown in FIG. 18, the dimension2*LETOE corresponds to the distance between the interior boundaries ofthe two signal effect sub-regions 50H1-SE and 50H2-SE, it should beappreciated that in an alternative embodiment, the relationship betweenthe sub-regions 50H1-SE and 50H2-SE may be such that the distancebetween their exterior boundaries (rather than their interiorboundaries) corresponds to the same dimension 2*LETOE. In either case,it should be appreciated that by selecting the edge-to-edge length LETOEaccording to the dimensional considerations and signal considerationsoutlined above in relation to EQUATIONS 1-5, a robust reference markprimary signal generating configuration is provided, especially when therelationships of EQUATIONS 2 and 3, or 4 and 5, are fulfilled. Thereference mark primary signals PREF1 and REF2 provided by the integratedsignal generating configuration 18000 exhibit a desirable reference markprimary signal relationship in a signal crossing region, as previouslyoutlined with reference to FIG. 14 and also with reference to FIG. 12.

In contrast to the reference signal generating configuration 16000, inthe reference signal generating configuration 18000 the reference marksecondary signal receiver channel apertures are located between thereference mark primary signal receiver channel apertures, along theX-axis direction. In such a case, in order to provide reliable referencemark secondary signals (e.g., SREF1, SREF1′, SREF2, and SREF2′),additional design relationships are desirable between the dimensionsand/or locations of the reference mark primary signal effect sub-regions50H1-SE and 50H2-SE, and the reference mark secondary signal receiverchannel apertures (e.g., as provided by the ends of the fibers 1890SR1,1890SR1′, 1890SR2 and 1890SR2′ in conjunction with the spatial filtermasks 1820SR1, 1820SR1′, 1820SR2 and 1820SR2′). In particular, EQUATIONS6 and 7 do not apply in this case. Instead, in this case it isadvantageous to arrange a reference signal generating configuration(e.g., the reference signal generating configuration 18000) such that:

ASR12SPAN≦2*LETOE  (Eq. 8)

This relationship corresponds to a configuration wherein the referencemark primary signal effect sub-regions 50H1-SE and 50H2-SE may bepositioned outside the reference mark secondary signal receiver channelapertures, such that they derive desirable signals from the interferencefringe light, without being significantly affected by the zero orderreflected light of the primary signal effect sub-regions 50H1-SE and50H2-SE. At the same time, the edges of the reference mark primarysignal effect sub-regions 50H1-SE and 50H2-SE may be centered relativeto the receiver channel apertures provided by the open aperture masks1820PR1 and 1820PR2 (e.g., as illustrated in FIG. 18), in order togenerate desirable reference mark primary signals (e.g., PREF1, andPREF2′) in a desirable primary signal crossing region, according topreviously described principles. In order to better insure suchdesirable signals, clearance dimensions PSCLR may be provided as shownin FIG. 18. By analogy with a previous description, PSCLR is theclearance from the edges of a reference mark primary signal effectregion (or sub-region) to the boundaries of the adjacent reference marksecondary signal receiver channel apertures when the reference markprimary signal effect region (e.g., the region 50H-SE) is nominallycentered relative to those boundaries along the measuring axis 82. Thus,in various embodiments analogous to that shown in FIG. 18:

ASR12SPAN=(2*LETOE)−(2*PSCLR)  (Eq. 9)

In various embodiments analogous to that shown in FIG. 18, it isdesirable for PSCLR to be greater than zero, or more desirably at least10 microns, and in some embodiments at least 25, and in otherembodiments at least 50 microns.

FIG. 19 is a diagram schematically illustrating various signalrelationships which may be associated with primary and secondaryreference signals according to this invention, as well as certainaspects of one method of associated signal processing. In particular,the extreme lower and upper portions of FIG. 19 reproduce the signalchart portions 1430 and 1440 of signal charts 60′ and 70′, respectively,and may be understood based on previous description. FIG. 19 also showsderived signal charts 1940 and 1930, and schematically represented logicsignals 1945 and 1935, described in greater detail below.

The signal chart 1940 illustrates a signal processed difference signalPDIFF, which is derived from the signals of the chart 1440 and which isequivalent to the difference between the combined reference mark primarysignals (PREF1 +PREF1′) and (PREF2 +PREF2′). Signal chart 1940 alsoshows a PDIFF upper threshold PUTR and a PDIFF lower threshold PLTRwhich define upper and lower reference signal levels that are comparedto the primary signal difference PDIFF. In one exemplary signalprocessing method, PUTR and PLTR are equally spaced relative to thePDIFF zero signal level 1921, which corresponds to the signal crossoverpoint 74′, which is taken as the primary reference position. In oneexemplary signal processing method, when the value of PDIFF is betweenPLTR and PUTR, the primary reference position indicator signal 1945 willbe switched to a high state 1945′, which indicates that a readheadaccording to this invention is positioned at a primary referenceposition proximate to a reference mark according to this invention,within a first uncertainty range and/or first resolution levelapproximately corresponding to the range between PLTR and PUTR.

The signal chart 1930 illustrates a signal processed difference signalSDIFF, which is derived from the signals of the chart 1430 and which isequivalent to the difference between the reference mark secondarysignals SREF1 and SREF2. Signal chart 1930 also shows an SDIFF upperthreshold SUTR and an SDIFF lower threshold SLTR which define upper andlower reference signal levels that are compared to the secondary signaldifference SDIFF. In one exemplary signal processing method, SUTR andSLTR are equally spaced relative to the SDIFF zero signal level 1920,which corresponds to the signal crossover point 64, which is taken asthe secondary reference position. In one exemplary signal processingmethod, when the value of SDIFF is between SLTR and SUTR, the secondaryreference position indicator signal 1935 will be switched to a highstate 1935′. It should be appreciated that a readhead and reference markscale according to this invention may be configured to provide theprimary reference position indicator signal high state 1945′ withsufficient resolution and repeatability that it may unambiguouslycorrespond to a single instance 1936 of the high state 1935′ of theprimary reference position indicator signal. Such a single instance ofthe high state 1935′ may indicate that a readhead according to thisinvention is positioned at a secondary reference position proximate to areference mark according to this invention, within a second uncertaintyrange and/or second resolution level approximately corresponding to therange between PLTR and PUTR. The second uncertainty range and/or secondresolution level may be significantly better than the first uncertaintyrange and/or first resolution level, according to previously describedprinciples. In various embodiments, the identification of a singleinstance 1936 of the high state 1935′ may be establish by logicoperations based on the states of the signals 1945 and 1935, using knowntechniques. In one embodiment, the correspondence may simply beindicated by both signals being simultaneously in their high states, asillustrated in FIG. 19. However, in other embodiments, thecorrespondence may be established based on more complicated processing(e.g., including processing based on the relationships between risingand falling edges of the signals 1945 and 1935, etc.). Thus, the methodand signals illustrated in FIG. 19 are exemplary only, and not limiting.

In one embodiment, the primary reference position may be establishedwith a resolution and accuracy on the order of 0.2 microns, based solelyon the signal 1945. The secondary reference position may then beestablished with a resolution and accuracy on the order of 20nanometers, based on using the signal 1935 in conjunction with thesignal 1945, as outlined above.

FIGS. 20A and 20B show illustrations 20000A and 20000B, respectively,which include alternative reference marks 50-M' and 50-M″, respectively.The reference mark structures 50-M' and 50-M″ are usable in place of thereference mark 50-M shown in FIG. 5, in various embodiments, and may begenerally understood by analogy. However, in contrast to the referencemark 50-M, the transverse edges of the reference marks 50-M' and 50-M″,which extend transverse to the measuring axis direction include portionsthat are angled relative to the Y-axis direction such that thetransverse edges are not straight. Stated another way, the transverseedges of each of the reference marks 50-M' and 50-M″ is configured suchthat different respective portions of a transverse edge have respectivelocations along the measuring axis direction that vary as a function ofposition along a direction transverse to the measuring axis direction.In particular, the respective locations along the measuring axisdirection, which vary as a function of position along a directiontransverse to the measuring axis direction, span a correspondingreference mark edge transition zone RMET along the measuring axisdirection. In various embodiments, it may be advantageous if therespective locations along the measuring axis direction vary back andforth repeatedly along the direction transverse to the measuring axis.In operation, such edge configuration may help suppresses edgediffraction effects that may otherwise arise in association withstraight edges. Such edge diffraction effects may add undesirableirregularities to the reference mark primary signals and/or thereference mark secondary signals. When such edge configurations areused, it is convenient to define the dimension LETOE as the distancefrom the middle of a first reference mark edge transition zones RMET tothe middle of an operationally corresponding second reference mark edgetransition zone RMET (e.g., approximately as illustrated by thedimensions LETOE′ and LETOT″ in FIGS. 20A and 20B). With the dimensionLETOE so defined, the reference mark may then be designed to satisfydesirable conditions outlined above with reference to EQUATIONS 1-9. Invarious embodiments, it may be advantageous if the transverse edges insuch first and second reference mark edge transition zones areconfigured as a minor image of the first reference mark portiontransverse edge with respect to a symmetry axis along a directionperpendicular to the measuring axis direction, such that the resultingreference mark primary signals which cross in the signal crossing regionmay also tend to have a minor image symmetry that is relatively robust.

It is evident that the embodiments and design factors described aboveare indicative of additional alternative embodiments, modifications andvariations, as will be apparent to those skilled in the art. As a firstexample, although the foregoing discussion describes embodiments thatinclude mirror-type reference mark portions that include planar mirrors,more generally “significant zero order reflection” reference markportions may be used in place of minor reference mark portions. Such“significant zero order reflection” portions may comprise anyarrangement of surfaces that provide a significant amount of zero orderreflected light, and/or that disturb a significant amount of +/−firstorder diffracted light, such that the corresponding reference marksignal level can be distinguished from the signal level resulting froman adjacent “zero order reflection suppressing” portion (e.g., a portionof an incremental measurement scale grating or a grating track portion).For example, in various embodiments, a “significant zero orderreflection” portion may comprise a phase grating with an 80-20 dutycycle (e.g., similar to the phase grating shown in FIG. 5, but withW_(E)=0.8*P_(g) and W_(G)=0.2*P_(g)), or a 70-30 duty cycle, etc. Invarious other embodiments, the zero order reflection portion maycomprise a 50-50 duty cycle phase grating, but with a grating bar heightthat does not suppress zero order reflection (e.g., similar to the phasegrating shown in FIG. 5, but with H_(E)=0.5*illumination wavelength orH_(E)=0.1*illumination wavelength, etc.). In other embodiments, one ormore grating elements of a zero order reflection portion may befabricated to have a different reflectance than other portions of thescale.

As a second example, it will be appreciated that in embodiments such asthose shown in FIGS. 6A, 6B and 7, the individual reference signals REF1and REF1′ are redundant, and the individual reference signals REF2 andREF2′ are redundant. Although certain advantages with regard to signalstrength and/or alignment sensitivity may be gained from suchredundancy, in general, in any embodiments disclosed herein that useredundant signals (including those that use redundant reference markprimary or secondary signals), redundant signals may be eliminated andthe associated optical fiber arrangements may consist of fewer opticalfibers and/or reference mark signal receiver channels, than thoseillustrated herein.

As a third example, although various embodiments of the invention havebeen illustrated using straight scale tracks, the same or similarembodiments may used with curvilinear or circular scale tracks (e.g., inrotary or angular encoders). Thus, in various embodiments, the termsscale track and measuring axis direction, for example, may beinterpreted as referring to a circular or curvilinear track or measuringaxis, and the related illustrations may be interpreted as showingtangential portions of such circular or curvilinear tracks or measuringaxes.

Furthermore, the various features such as angled transverse edges forapertures and/or reference marks have been described in the context ofconfigurations that provide both reference mark primary signals andreference mark secondary signals. However, such features may alsoprovide benefits when used in configurations that provide only referencemark primary signals (e.g., the various configurations described withreference to FIGS. 6A-12). Accordingly, the embodiments of theinvention, as set forth above, are intended to be illustrative, notlimiting.

Enhanced Integrated Reference Mark Signal Generating Configurations

In some embodiments and/or applications, the reference mark signalgenerating configurations outlined above that provide reference markprimary signals and reference mark secondary signals in order to providean improved level of reference mark position resolution andrepeatability may not be desirable due to complexity, size, and/or costconsiderations. Also, in some embodiments and/or applications, it may bedesirable to provide an improved level of robustness and/or accuracy incomparison to reference mark signal generating configurations outlinedabove that provide only a primary or single set of reference marksignals. In particular, in embodiments where an integrated fiber opticreadhead and an integrated scale track provide both the referenceposition indication and the periodic incremental measurement signals, itis generally desirable to simultaneously satisfy two criteria. The firstcriterion is that the reference mark should provide a strong andlocalized reference mark signal light component in the reference marksignal effect region, in order to provide a satisfactory reference marksignal. However, at the same time, the reference mark should notsignificantly disrupt the spatially periodic incremental measurementsignals. The inventor has found that when the reference marks outlinedabove, or other prior art reference marks, are embedded in the scalegrating 80, while performance may be acceptable for some application,these dual criteria are not satisfied at a level that provides thehighest desirable levels of robustness and measurement resolution forall applications.

This problem is particularly difficult to solve because the receivingarea constraints in an optical fiber readhead are significant, limitingthe available design options (e.g., in comparison to the flexibility ofdesign afford by a photodetector in an electro-optic readhead, forexample). However, the inventor has found that an enhanced “zonegrating” reference mark structure described below, in combination withcertain aperture configurations described below, can satisfy these dualcriteria in an integrated fiber optic readhead and scale trackconfiguration, even for extremely fine pitch, high resolution (e.g.,sub-micron resolution) optical fiber readheads and scales.

FIG. 21 is an isometric view of an enhanced integrated miniature fiberoptic readhead and scale arrangement 21000 according to this invention,which includes an enhanced integrated incremental signal and referencesignal generating configuration. The operation of the enhancedintegrated miniature fiber optic readhead and scale arrangement 21000 isin some respects similar to that of miniature fiber optic readhead andscale arrangement 3000 of FIG. 3, and similarly numbered components(e.g., components numbered with similar “suffixes” such as 395 and 2195)may be similar or identical in form and operation, except as otherwiseindicated below. As shown in FIG. 21, the enhanced integrated miniaturefiber optic readhead and scale arrangement 21000 includes a scale member81 that has a scale track 85 that includes a scale grating 80 and areference mark zone 2151 that includes an enhanced zone gratingreference mark 50ZRM, described in greater detail below. The enhancedintegrated miniature fiber optic readhead and scale arrangement 21000also includes an integrated incremental and reference mark readhead2100, also referred to simply as an integrated readhead 2100. As shownin FIG. 21, an orthogonal XYZ coordinate system may be defined such thatthe Y-axis is parallel to the bars of the scale grating 80, the Z-axisis normal to the surface of the scale grating 80, and the X-axis isorthogonal to the Y-Z plane. A measuring axis direction 82 is parallelto the X-axis.

The scale grating 80, which is a first type of track portion of thescale track 85, extends over a measurement range along the measuringaxis direction 82. The scale grating 80 comprises a reflective periodicphase grating having a grating pitch PG, and is configured to suppresszero-order reflection. A second type of track portion, the enhanced zonegrating reference mark 50ZRM, is integrated with the scale grating 80 inthe scale track 85, in a configuration that can simultaneously provideboth incremental measurement signals and reference mark signals, with aminimum of crosstalk or interference between the signals, as describedin greater detail below. The enhanced zone grating reference mark 50ZRMmay include at least one zone grating reference mark portion (e.g., oneor two portions, in various embodiments).

The integrated readhead 2100 comprises a ferrule 2101 that houses andpositions the ends of a plurality of optical fibers 2130 that areincluded in a fiber-optic cable 2195, and a mask element 2161. Invarious embodiments, the integrated readhead 2100 may be configuredsimilarly to the various types of integrated readheads described herein.In various embodiments the integrated readhead 2100 may be aninterferometric-type readhead, which is configured to generateincremental measurement signals according to principles briefly outlinedherein, and described in greater detail in the incorporated '696 patent.In some embodiments, it may be particularly advantageous if theintegrated readhead 2100 is configured to include features describedbelow with reference to the integrated incremental signal and referencesignal generating configurations shown in FIGS. 23, 24, 27A, 27B and 28,or the like.

In operation, the scale member 81 displaces along the measuring axis 82such that the readhead 2100 is displaced along the scale track 85.

Briefly, in operation, the scale member 81 and the integrated readhead2100 are operably positioned relative to one another, such that aspecified operating gap ZGAP is provided between scale grating 80 and areceiver plane 2160 of the integrated readhead 2100. When the scalemember 81 and the readhead 2100 undergo relative motion along themeasuring axis direction 82, the readhead 2100 is displaced along scaletrack 85. The integrated readhead 2100 outputs a diverging source light2150 from the central one of the optical fibers 2130, which illuminatesthe scale track 85 at an illumination spot 2153. In various embodiments,the source light 2150 is advantageously monochromatic and spatiallycoherent, and may be temporally coherent in some embodiments. The sourcelight 2150 is generally reflected and diffracted to provide scale light2155.

The scale light 2155 may be described as comprising two types of scalelight components, a first incremental signal light component whichincludes a spatially periodic intensity pattern, and a second referencemark signal light component which includes a non-periodic concentratedlight intensity contribution arising from the zone grating referencemark 50ZRM. For example, regarding the first incremental signal lightcomponent, the periodic structure of the scale grating 80 gives rise to+/−first-order diffracted lights 2157 that are reflected to the readhead2100, as schematically indicated by diverging dashed lines in FIG. 21.The +/−first-order diffracted lights 2157 form a field of interferencefringes proximate to a receiver plane 2160 of the mask element 2161,which periodically spatially filters the interference fringes usingphase mask portions having different spatial phases over the ends ofcertain ones of the outer optical fibers 2130, in order to provide aplurality of fiber optic incremental measurement signal receiverchannels according to principles described previously herein, as well asin the incorporated '696 patent. As a result of the periodic spatialfiltering, certain fiber-optic receiver channels of the integratedreadhead 2100 provide incremental measurement signal receiver channelsthat may output periodic optical signals having different spatial phases(e.g., quadrature signals) when the scale grating 80 is displacedrelative to the readhead 2100. Regarding the second reference marksignal light component which includes a non-periodic concentrated lightintensity contribution arising from the zone grating reference mark50ZRM, in various embodiments when the zone grating reference mark 50ZRMis located in the illumination spot 2153, the zone grating markportion(s) produces a combination of light from at least two referencemark zones that constructively combine to provide a concentrated orfocused scale light component in a zone grating reference mark signaleffect region 50ZRM-SE at or proximate to the receiver plane 2160. (InFIG. 21, the zone grating reference mark signal effect region 50ZRM-SEis schematically shown at an exaggerated distance from the receiverplane 2160, for purposes of illustration.) In the embodiment shown inFIG. 21, the mask element 2161 of the integrated readhead 2100 alsoincludes regions or aperture masks that provide no periodic spatialfiltering over the ends of certain ones of the outer optical fibers2130, such that the spatially periodic effects of the incremental signallight component are rejected or suppressed, and the second referencemark signal light component in the zone grating reference mark signaleffect region 50ZRM-SE provides a signal change that is sensed by thecorresponding reference mark signal receiver channels. In particular,when the readhead 2100 is displaced over the zone grating reference mark50ZRM, the relatively constant amount of “averaged” first incrementalsignal light component remains nearly unchanged and the concentratedreference mark signal light component in the zone grating reference marksignal effect region 50ZRM-SE provides a change in the signal that isreceived and transmitted as a reference signal by the reference marksignal receiver channels to provide a plurality reference mark signalsthat are modulated as a function of the amount of overlap of theillumination spot 2153 and the zone grating reference mark 50ZRM. Theplurality reference mark signals are processed such that a referenceposition can be precisely determined, as described in greater detailbelow.

As previously indicated, it is essential for the integrated readhead2100 to continue to output the periodic optical signals having differentspatial phases that are used for incremental displacement measurement(e.g., the incremental measurement quadrature signals), simultaneouslywith outputting the reference mark optical signals, when illuminationspot 2153 of the integrated readhead 2100 passes over the reference markzone 2151. Furthermore, particularly when using a fine pitch scalegrating 80, it is advantageous to enhance the fidelity of theincremental displacement measurement signals (e.g., so that they can beaccurately interpolated to provide sub-micron resolution), and thesignal to noise ratio of the reference mark signals (e.g., so that theycan reliably and robustly indicate a reference location within singlespecific period of the scale grating 80), as much as is practical. Forthis purpose, has been found that it is advantageous that the zonegrating reference mark 50ZRM satisfy various design considerationsbeyond those recognized in the prior art, as described in greater detailbelow.

FIG. 22 is an isometric view showing a generic integrated scale gratingand zone grating reference mark structure 2200 usable in variousembodiments according to this invention. Components numbered similarlyto those shown in FIG. 21 may be similar or identical in someembodiments. A portion of a typical zone grating reference mark 50ZRM isshown in FIG. 22. It will be understood that in various embodiments itmay be advantageous for the zone grating reference mark 50ZRM to besymmetrical about a centerline ZRMC shown in FIG. 22.

As shown, a scale track 85 located on a scale member 81 includes theintegrated scale grating and zone grating reference mark structure 2200,which comprises at least a portion of the scale grating 80 and the zonegrating reference mark 50ZRM. The zone grating reference mark 50ZRM isschematically illustrated as a generic example (e.g., the number ofelements in various zones is only a symbolic representation, and thedrawing is not to scale). Both the scale grating 80 and the zone gratingreference mark 50ZRM comprise reflective phase grating elements, theraised elements E and the recessed elements G, which may be formed byknown methods. Briefly, in some embodiments, the reflective phasegrating elements E and G may be formed with a reflective chrome coating,and with a recess height HE between the raised elements E and therecessed elements G, which causes destructive interference between lightreflected from the elements E and G and tends to suppresses zero orderreflected light. For example, a height HE of (¼+N/2) wavelengths (whereN=0, 1, 2, etc.) of the source light may be used.

The raised and recessed elements of the scale grating 80 are designatedES and GS, respectively. The raised scale elements ES are centered atcorresponding raised grating element nominal locations EL, wherein theraised grating element nominal locations are spaced apart from oneanother by the scale grating pitch PG along the measuring axisdirection. Similarly, the recessed scale elements GS that are centeredat corresponding recessed grating element nominal locations GL, and therecessed grating element nominal locations GL are spaced apart from oneanother by the grating pitch PG along the measuring axis direction, andoffset from the raised grating element nominal locations EL by one halfthe grating pitch PG along the measuring axis direction 82. In variousembodiments, it is advantageous if the scale elements EL are centered atthe raised grating element nominal locations EL, and the recessed scaleelements GS are centered at the recessed grating element nominallocations GL. In various embodiments, the widths WES and WEG of theraised elements ES and the recessed elements GS may be approximatelyequal (also referred to as a 50:50 duty cycle), in order tosubstantially eliminate zero order reflection and all even diffractionorders from the scale grating 80. However, in some embodiments, slightdeviations from a 50:50 duty cycle may compensate for process inducedprofile aberrations, and provide better zero order reflectionsuppression from the scale grating 80, as may be determined by analysisand/or experiment.

The raised and recessed elements of the zone grating reference mark50ZRM are designated EZ and GZ, respectively. In addition, a suffix maybe added to EZ and GZ (e.g., EZ1, GZ1) to designate the zone that theyare located in. The zones are described further below. As shown in FIG.22, the raised grating element nominal locations EL and the recessedgrating element nominal locations GL are defined to continue from thescale grating 80 along the measuring axis direction 82 and through thezone grating reference mark 50ZRM. Because of variations in the widthsof the scale elements in the various zones, as described further below,the raised scale elements EZ always span corresponding raised gratingelement nominal locations EL, and may be close to centered, but may notbe perfectly centered at the nominal locations EL in some embodiments.Similarly, because of variations in the widths of the scale elements inthe various zones, the recessed scale elements GZ always spancorresponding recessed grating element nominal locations GL, and may beclose to centered, but may not be perfectly centered at the nominallocations GL in some embodiments. However, in some embodiments, at leastone of the raised or recessed types of elements EZ or GZ may always becentered at the corresponding nominal locations EL or GL, respectively.For example, FIG. 22 shows each of the raised elements EZ centered atthe corresponding nominal locations EL.

The widths WEZ and WGZ of the raised elements EZ and the recessedelements GZ are generally not equal in the zone grating reference mark50ZRM. They are unequal to a carefully determined extent, in that thezone grating reference mark 50ZRM must generally satisfy two criteria.Of course the first criteria is that the zone grating reference mark50ZRM should provide a sufficient concentrated reference mark signallight component in the zone grating reference mark signal effect region50ZRM-SE, in order to provide a satisfactory reference mark signal, aspreviously outlined. However, at the same time, the zone gratingreference mark 50ZRM should not significantly disrupt the spatiallyperiodic incremental measurement signals. The inventor has found thatprior art zone plate reference marks cannot be embedded in the scalegrating 80 and satisfy these dual criteria. In contrast, the illustratedzone grating reference mark structure in combination with certainaperture configurations described further below can satisfy these dualcriteria, even for extremely fine pitch, high resolution (e.g.,sub-micron resolution) optical fiber encoders. In particular, withproperly established zone boundaries (e.g., the zone outer boundariesZONE1OB, ZONE2OB, and ZONE3OB, shown in FIG. 22) the inequality of thewidths WEZ and WGZ may be chosen (e.g., as determined by analysis ofexperiment) to satisfy these dual criteria. It will be appreciated thatto the extent that the widths WEZ and WGZ match, the matching portionswill suppress a certain amount of zero order reflected light, andcontribute to odd-order diffracted light, similar to the scale grating80. Thus, a zone grating reference mark 50ZRM according to thisinvention may partially contribute to the interference fringes thatprovide the incremental measurement signals, and avoid significantdisruption of such signals. Conversely, to the extent that the widthsWEZ and WGZ are unmatched, the unmatched portions will contribute “zeroorder” reflected light. In terms of zone plate operation, such light maybe understood in terms of the well known Huygens-Fresnel principle.Thus, it may be understood by one of ordinary skill in the art havingthe benefit of this disclosure, that the unmatched portions of thewidths WEZ and WGZ may contribute light that behaves according to knownzone plate principles. Thus, a zone grating reference mark 50ZRMaccording to this invention may contribute light from various zones thatconstructively and destructively combines at certain locations such thatit, in effect, provides a concentrated or focused scale light componentin the zone grating reference mark signal effect region 50ZRM-SE at orproximate to the receiver plane 2160, as outlined above. The zonegrating reference mark signal effect region 50ZRM-SE may generally benarrower than the zone grating reference mark 50ZRM. However, in anycase, the zone grating reference mark 50ZRM and its correspondingreference mark signal effect region 50ZRM-SE have a repeatablerelationship at positions along the measuring axis direction, and thereference mark signal receiver channel apertures receive detectablydifferent amounts of scale light depending on their proximity with thezone grating reference mark 50ZRM and its corresponding reference marksignal effect region 50ZRM-SE.

As previously indicated, in various embodiments according to thisinvention, the inequality of the widths WEZ and WGZ may be chosen toprovide a desired intensity for the focused scale light component, whilemaintaining a desirable spatially periodic light intensity arising frominterfering +/−first order diffracted scale light, as required for theincremental measurement signals. Zone grating reference mark design isdescribed in greater detail below with reference to FIG. 26. Briefly, insome embodiments, the widths WEZ and WGZ may correspond to them being onthe order of PG/2+/−25% of PG, or PG/2+/−15% of PG, or PG/2+/−5% of PG,or the like. In some embodiments, it has been found advantageous if thewidths WEZ and WGZ vary from PG/2 by at most +/−25% of PG and at least+/−5% of PG, or vary from PG/2 by at most +/−20% of PG and at least+/−10% of PG in other embodiments. For example, in one embodiment,symbolically represented in FIG. 22, in ZONE1, WEZ1=0.65*PG andWGZ1=0.35*PG (that is, a 65:35 duty cycle). In ZONE2, in order toproduce a scale light component that is 180 degrees out of phase withthe scale light component from ZONE1, WEZ11=0.35*PG and WGZ1=0.65*PG(that is, a 35:65 duty cycle). In ZONE3, in order to produce a scalelight component that is in phase with the scale light component fromZONE1, again WEZ1=0.65*PG and WGZ1=0.35*PG (that is, a 65:35 dutycycle). It will be appreciated that the foregoing values for the widthsWEZ and WGZ are exemplary only, and not limiting. More generally, insome embodiments, the widths WEZ need not be constant throughout a zone,and the widths WGZ need not be constant throughout a zone. Or statedanother way, the duty cycle need not be constant throughout a zone. Insuch embodiments, if the widths WEZ and WGZ are characterized by theiraverage values within a zone, those average values may exhibit adifference or a deviation from PG/2 approximately as outlined above forthe values WEZ and WGZ.

In addition, the locations of the outer zone boundaries ZONE1OB,ZONE2OB, and ZONE3OB are chosen such that optical path lengths (e.g.,the average optical path length) from each of the zones causes theirlight to constructively interfere at the reference mark signal effectregion 50ZRM-SE, as described further below with reference to FIG. 26.In the embodiment shown in FIG. 22, the grating element positioned ateach zone boundary has a dimension of 0.5*PG (e.g., the same as acorresponding element of the scale grating 80), and the adjacentelements have widths which allow a transition to the typical duty cyclesof the adjacent zones. However, it will be appreciated that this aspectof the configuration shown in FIG. 22 is exemplary only, and notlimiting. It should be appreciated that in some alternative embodiments,the zone grating reference mark 50ZRM may comprise two zones, oradditional zones. One such embodiment is illustrated in FIG. 26. Zoneboundary locations, the numbers of elements in each zone, and so on, aredescribed further below with reference to FIG. 26.

FIG. 23 is an isometric view schematically showing various aspects of anintegrated incremental signal and reference signal generatingconfiguration 23000 (also referred to as an integrated signal generatingconfiguration 23000), which includes an integrated scale grating andzone grating reference mark arrangement, usable in various embodimentsaccording to this invention. Components numbered similarly to thoseshown in FIGS. 21 and 22 (e.g., with similar suffixes, such as 2350 and2150) may be similar or identical in some embodiments, and maybeunderstood by analogy. The view shown in FIG. 23 does not show a maskelement (e.g., such as the mask element 2161, shown in FIG. 21), whichis an essential element of the integrated incremental signal andreference signal generating configuration 23000, in order to moreclearly illustrate other aspects of its operation. A mask element 2361is described below with reference to FIG. 24.

FIG. 23 shows certain elements of the integrated signal generatingconfiguration 23000, including an integrated readhead optical fiberarrangement 2300 (less the mask element 2361, shown in FIG. 24) operablypositioned relative to a schematically represented zone gratingreference mark 50ZRM surrounded by the scale grating 80 (e.g., on ascale member 81, not shown). The reference mark 50ZRM has a dimensionLETOE(ZRMSE) along the X-axis direction, and a centerline ZRMC, asshown. Briefly, in operation, the integrated readhead optical fiberarrangement 2300 outputs a diverging source light 2350 from a lightsource 2380 provided by a central fiber, which illuminates the zonegrating reference mark 50ZRM and the surrounding scale grating 80 at theillumination spot 2353. In various embodiments, the source light 2350 isadvantageously monochromatic and spatially coherent, and may betemporally coherent in some embodiments. The scale grating 80, whenilluminated, suppresses zero order reflected light and providesreflectively diffracted +/−first order scale light 2355A and 2355B,schematically represented by dashed lines representing angled diffractedorders in FIG. 23. As previously outlined, to the extent that the widthsof the raised and recessed elements match in the zone grating referencemark 50ZRM match, the matching portions will suppress a certain amountof zero order reflected light, and contribute a certain amount of lightin the zone grating reference mark scale light 2345 to the reflectivelydiffracted +/−first order scale light 2355A and 2355B, similar to thescale grating 80.

The +/−first-order scale lights 2355A and 2355B illuminate regions2355A′ and 2355B′, respectively, which overlap in an interference zone2356 to form interference fringes 2366 proximate to a receiver plane2360 of the integrated readhead optical fiber arrangement 2300. Theinterference fringes are spatially filtered by the mask element 2361(shown in FIG. 24) over the ends of the optical fibers that receive theoptical quadrature signals A, A−, B and B−, according to previouslydescribed principles, and as described in greater detail below withreference to FIG. 24.

Simultaneously with the incremental quadrature signal generatingoperations outlined above, when the zone grating reference mark 50ZRMfalls within the illumination spot 2453, it also reflects a portion ofthe diverging source light 2350 to provide the reference mark signaleffect region 50ZRM-SE, having the dimension LETOE(ZRMSE) along theX-axis direction. As previously outlined, to the extent that the widthsof the raised and recessed elements in the zone grating reference mark50ZRM are unmatched, the unmatched portions will contribute “zero order”reflected light. In terms of zone plate operation, such light may beunderstood in terms of the well known Huygens-Fresnel principle. Thus,it may be understood by one of ordinary skill in the art having thebenefit of this disclosure, that the unmatched portions of the raisedand recessed element widths may contribute light that behaves accordingto known zone plate principles. Thus, the zone grating reference mark50ZRM contributes a certain amount of light in the zone gratingreference mark scale light 2345 from the zones ZONE1, ZONE2, and ZONE3,that constructively and destructively combines at certain locationsalong the receiver plane 2360 such that it, in effect, provides aconcentrated or focused scale light component in the zone gratingreference mark signal effect region 50ZRM-SE at or proximate to thereceiver plane 2360.

As previously outlined, the zone grating reference mark signal effectregion 50ZRM-SE may generally be narrower along the X-axis directionthan the zone grating reference mark 50ZRM, while having a repeatablerelationship to the zone grating reference mark 50ZRM at positions alongthe measuring axis direction. In some embodiments, the zone gratingreference mark 50ZRM may be symmetric about its center line ZRMC, andthe center line ZRMC-SE of the zone grating reference mark signal effectregion 50ZRM-SE may approximately align with the center line ZRMC whenthe zone grating reference mark 50ZRM is approximately centered in theillumination spot of the readhead. However, in some embodiments, thezone grating reference mark 50ZRM may be asymmetric, and the centerlines ZRMC and ZRMC-SE may not align, although they will still maintaina repeatable relationship relative to one another at positions along themeasuring axis direction. In any case, the light in the zone gratingreference mark signal effect region 50ZRM-SE is received by referencesignal aperture masks that are located at the receiving plane 2360 tomask the ends of the optical fibers that receive the reference signalsREF1 and REF2, such that the reference signals REF1 and REF2 have adesired relationship for indicating a reference position within a signalcrossing region proximate to the zone grating reference mark 50ZRM, asdescribed in greater detail below with reference to FIGS. 24 and 25.

As previously outlined, and as schematically shown in FIG. 23, scalelight in the zone grating reference mark signal effect region 50ZRM-SEmay be understood as comprising two components. The two components arean incremental signal light component ISLCOMP, which includes a periodicintensity distribution as shown in FIG. 23, and a reference mark signallight component RMSLCOMP, which includes a substantially non-periodicintensity distribution as shown in FIG. 23. The reference signalaperture masks may be configured to average the spatially periodicintensity incremental signal light component ISLCOMP, such that itbecomes an approximately constant offset component that is shared by thereference signals REF1 and REF2. The reference mark signal lightcomponent RMSLCOMP may comprise an elevated light intensity distributionacross the reference mark signal effect region 50ZRM-SE, which may havea maximum or peak value within the zone grating reference mark signaleffect region 50ZRM-SE, and diminishes away from that maximum value. Insome embodiments, the two signal effect region boundaries SERBL andSERBR, which correspond to the effective edge-to-edge dimensionLETOE(ZRMSE) of the zone grating reference mark signal effect region50ZRM-SE along the measuring axis direction 82, may be defined tocorrespond to the full width half maximum dimension of the intensitydistribution of the reference mark signal light component RMSLCOMP.However, more generally, in some embodiments, each of the two signaleffect region boundaries SERBL and SERBR may be defined to fall within arange where the intensity contribution of the reference mark signallight component of the scale light is at most 80% of its maximumcontribution in the elevated intensity distribution and at least 20% ofits maximum contribution in the elevated intensity distribution. Evenmore generally, in some embodiments, the effective edge-to-edgedimension LETOE(ZRMSE) of the zone grating reference mark signal effectregion 50ZRM-SE may be operationally defined and/or adjusted by thedesign of the zone grating reference mark 50ZRM, such that itcorresponds to the locations bounding the portion of the intensitydistribution of the reference mark signal light component RMSLCOMP thathave a significant effect on the reference signals REF1 and REF2 withrespect to determining their crossover at the reference position in asignal crossing region, as described in greater detail below withreference to FIG. 25.

FIG. 24 is an isometric view schematically showing various additionalaspects of a portion 23000′ of the integrated incremental signal andreference signal generating configuration 23000 shown in FIG. 23,including the integrated readhead optical fiber arrangement 2300, thereference mark signal effect region 50ZRM-SE, and a mask element 2361.Components numbered similarly to those shown in FIGS. 21-23 (e.g., withsimilar suffixes, such as 2350 and 2150) may be similar or identical insome embodiments, and maybe understood by analogy. As shown in FIG. 24,the integrated readhead optical fiber arrangement 2300 may comprise acentral source fiber 2370 that provides the light source 2380, which maybe provided by the end of a single-mode core of the source fiber 2370 insome embodiments, and receiver fibers 2390A, 2390A′, 2390B, 2390B′,2390R1, and 2390R2, that provide receiver channels that receive theoptical signals A, −A, B, −B, REF1, and REF2, respectively, as shown.The integrated readhead optical fiber arrangement 2300 also includes amask element 2361 comprising phase masks 2320A, 2320B, 2320A′, 2320W,blocking mask portions 2320BR1 and 2320BR2, and open reference signalaperture mask portions 2320R1 and 2320R2 (also referred to simply asreference signal apertures 2320R1 and 2320R2). Dashed lines ARIL, AR1Rand CAR1 show the positions of the left and right boundaries andeffective center location, respectively, of the reference signalaperture 2320R1 corresponding to the signal REF1, and dashed lines AR2L,AR2R and CAR2 show the positions of the left and right boundaries andeffective center location, respectively, of the reference signalaperture 2320R2 corresponding to the signal REF2. The dimensionLCAR1CAR2 denotes the distance along the X-axis between the effectivecenters of the reference signal apertures corresponding to signals REF1and REF2. The dimension AR12SPAN denotes the total distance spannedbetween the boundaries ARIL and AR2R. The dimension AR12SEP denotes thedistance between the boundaries AR1R and AR2L, and is zero in theembodiment shown in FIG. 24.

Briefly, in operation, the phase masks 2320A, 2320B, 2320A′, and 2320B′are located at the receiving plane 2360, and are configured according toknown principles to spatially filter the interference fringes having thefringe pitch FP in the interference zone 2356 to provide quadrature-typeperiodic incremental measurement signals A, A′, B, and B′, respectively.In one embodiment, the phase masks 2320A, 2320B, 2320A′, and 2320B′ haverelative spatial phases of 0, 90, 180 and 270 degrees, respectively. Itwill be appreciated that the relative positions of the measurementsignals A, A′, B, and B′, are illustrative only, and not limiting. Ingeneral, the phase masks may be configured to provide the respectivemeasurement signals A, A′, B, and B′ from any one of the respectiveoptical fibers 2390A, 2390A′, 2390B and 23980B′ that is desired. Variousoperation and design principles that are relevant to the mask element2361, as well as alternative spatial phase arrangements, are describedelsewhere herein or in the incorporated references.

Blocking mask portions 2320BR1 and 2320BR2 and reference signalapertures 2320R1 and 2320R2 are located at the receiving plane 2360 tomask the ends of fibers 2390R1 and 2390R2 and provide reference signalsREF1 and REF2. It will be appreciated that in the absence of theblocking masks 2320BR1 and 2320BR2 the ends of the receiver fibers2390R1 and 2390R2, which are collocated along the direction of themeasuring axis 82, would respond to some positions of the reference marksignal effect region 50ZRM-SE with identical signals. In contrast, thearrangement of the masks 2320BR1, 2320BR2, 2320R1 and 2320R2 providesreference mark signal receiver channel apertures that are offset alongthe direction of the measuring axis 82, to provide reference signalsREF1 and REF2 that exhibit a desirable signal crossing region, asdescribed in greater detail below.

As previously indicated, the interference fringes arising in theinterference zone 2356 are present continuously during the operation ofthe integrated signal generating configuration 23000. However, incontrast to the phase masks, the reference signal apertures 2320R1 and2320R2 provide no periodic spatial filtering over the ends of receiverfibers 2390R1, and 2390R2, and the light from a plurality of fringessimply provides a relatively constant average amount of light to thereference signals REF1 and REF2, independent of displacement. Incontrast, when the reference mark signal effect region 50ZRM-SE overlapsthe locations of the reference signal apertures 2320R1 and 2320R2, itsfocused reference mark scale light significantly increases the referencesignals REF1 and REF2, as a function of the amount of overlap.

The amount of overlap between the reference mark signal effect region50ZRM-SE and the reference signal apertures 2320R1 and 2320R2 at variouspositions governs the relationship between the reference signals R1 andR2 at those positions. For the integrated incremental signal andreference signal generating configuration 23000, the most general designguidelines to provide appropriate reference signals R1 and R2 are thatthe integrated incremental signal and reference signal generatingconfiguration 23000 and the zone grating reference mark 50ZRM should beconfigured such that such that the following relationship is fulfilled:

AR12SEP<(LETOE(ZRM−SE))<AR12SPAN  (Eq. 10)

and such that the resulting reference mark signals are usable to definea reference position with a desired accuracy and/or repeatability withina signal crossing region proximate to the zone grating reference mark50ZRM, as described in greater detail below. In various embodiments, aconfiguration that furthermore fulfills the relationships

(LETOE(ZRMSE))>[AR12SEP−F(0.25*(AR12SPAN−AR12SEP))]

(LETOE(ZRMSE))<[AR12SEP−F(0.75*(AR12SPAN−AR12SEP))]  (Eqs. 11 & 12)

may be advantageous (e.g., by providing a more robust and/or reliablerelationship between the reference mark signals). In various otherembodiments, a configuration that furthermore fulfills the relationships

(LETOE(ZRMSE))>[AR12SEP+(0.4*(AR12SPAN−AR12SEP))]

(LETOE(ZRMSE))<[AR12SEP+(0.6*(AR12SPAN−AR12SEP))]  (Eqs. 13 & 14)

may be more advantageous. In some embodiments, it may be mostadvantageous if the dimension LETOE(ZRMSE) is approximately equal to[AR12SEP+(0.5*(AR12SPAN−AR12SEP))], or approximately equal to theeffective center to center distance LCAR1CAR2 between the reference marksignal receiver channel apertures corresponding to the signals REF1 andREF2, to provide reference mark signals approximately as described belowwith reference to FIG. 25.

According to previously outlined principles, the reference signalapertures 2320R1 and 2320R2 provide no periodic spatial filtering overthe ends of corresponding optical fibers 2390R1 and 2390R2, such thatthe spatially periodic effects of the incremental signal light componentare averaged to a value that is approximately constant withdisplacement, while the reference mark signal light component in thezone grating reference mark signal effect region 50ZRM-SE provides asignal change that is sensed by the corresponding reference mark signalreceiver channels. In the embodiment shown in FIG. 24, the blocking mask2320BR1, open aperture mask 2320R1, blocking mask 2320BR2 and openaperture mask 2320R2, provide left and right aperture edges that extendtransverse to the measuring axis direction, and those transverseaperture edges include zigzagging portions. The zigzagging portions areangled relative to the Y-axis such that each transverse aperture edgezigzags to span a primary aperture edge transition dimension PAETextending from an adjacent aperture boundary (e.g., the boundaryindicated by the dashed line AR2R) along the direction of the measuringaxis 82 toward the opposite edge of the aperture. A representativedimension PAET is shown only for the right edge of the open aperturemask 2320R2, however, it will be understood that analogous dimensionsPAET exist for each left and/or right aperture edge in FIG. 24. Thedimensions PAET need not be the same for each aperture edge, althoughthey may be, in various embodiments. By angling the zigzagging portionsof the respective aperture edges to span their respective dimensionsPAET, any interference fringes that pass across the open aperture masks2320R1 and/or 2320R2 are spatially filtered at the edges of theapertures such that the amplitude of their disruptive periodic opticalsignal contribution to the reference mark signals R1 and R2 are at leastpartially suppressed. It may be advantageous, in various embodiments, ifeach dimension PAET is at least one fringe pitch FP at the receivingplane 2360 of the mask element 2361. In some embodiments, each dimensionPAET may be at least three times FP, or more. In some embodiments, whilenot necessary, it may be advantageous if each dimension PAET isnominally equal to an integer number of fringe pitches FP at thereceiving plane of the mask element. It will be appreciated that thefringe pitch FP shown in FIG. 24 is not necessarily to scale and may beexaggerated for purposes of illustration.

According to a separate aspect of the configuration shown in FIG. 24, insome embodiments, while not necessary, it may be advantageous if aprimary aperture width dimension PAW is nominally equal to an integernumber of fringe pitches FP at the receiving plane of the mask element,at least along a majority of the length of the apertures edges along theY-axis direction. In such a case, the disruptive optical signalcontribution of any fringes that cross the reference signal apertures2320R1 or 2320R2 will tend to be more constant, particularly in thevicinity of the primary signal crossing region, which is less disruptiveto the reference mark primary signal crossing position than a variablesignal contribution from the fringes. The primary aperture widthdimension PAW may vary along the Y-axis direction or it may be constant.In any case, it may be defined at each location along the Y-axis, as theopen aperture dimension along the direction of the measuring axis 82.This design feature may be used in combination with a non-zero primaryaperture edge transition dimension PAET (e.g., as shown in FIG. 24), orit may also be beneficial in combination with straight aperture edgesthat are aligned along the Y-axis direction.

In various embodiments, it may be advantageous to configure anintegrated readhead optical fiber arrangement such that all opticalfibers located within a fiber optic readhead are located within acylindrical volume having a diameter of at most 1.5 millimeters, or 1.0millimeters, or less. In one specific embodiment, the fibers 2390 mayhave light carrying core areas having diameters DRA of approximately 200microns, outer diameters DRF of approximately 250 microns and thecentral fiber 2370 may have the same outer diameter DRF, and asingle-mode core diameter, or mode-field diameter, of approximately 4-10microns. Accordingly, in such an embodiment, the integrated readheadoptical fiber arrangement 2300 may have an overall diameter on the orderof 750 microns. However, it will be appreciated that in otherembodiments, larger or smaller fibers and/or other fiber spacings may beused.

In some embodiments, the grating pitch PG may be approximately 4 microns(or more generally, in the range of 1-8 microns), and the interferencefringes 2366 may have a similar pitch. The reference mark signalreceiver channel apertures 2320R1 or 2320R2 may have a dimension on theorder of 70 microns along the direction of the measuring axis 82 (e.g.,the dimension PAW). The reference mark 50ZRM may have a lengthLETOE(ZRM) that is advantageously on the order of approximately 142microns in one embodiment, which may result in an effective dimensionLETOE(ZRMSE) of approximately 70-80 microns for the reference marksignal effect region 50ZRM-SE, which provides a desirable tradeoffbetween adequate reference mark signal strength and minimal disturbanceto the incremental measurement signals. However, it should beappreciated that the particular embodiments of the dimensionalrelationships outlined above are exemplary only, and not limiting. Forexample, in various applications, additional design considerations(e.g., various values of the grating pitch PG) may favor smaller orlarger dimensions for LETOE(ZRM) and/or LETOE(ZRMSE), and the variousreference signal aperture dimensions outlined above.

FIG. 25 is a diagram showing a schematic signal chart 70″ illustratingthe reference signals generated corresponding to the integratedincremental signal and reference signal generating configuration 23000of FIGS. 23 and 24. The signal chart 70″ shows two reference signals, asignal REF1 and a signal REF2, as a function of relative position alongthe measuring axis 82 between the reference mark signal effect region50ZRM-SE (or the reference mark 50ZRM), and the integrated readheadoptical fiber arrangement 2300. In particular, the point 71″ correspondsto a position where the center line ZRMCSE of the reference mark signaleffect region 50ZRMSE coincides with a position displaced by the length0.5*LETOE(ZRMSE) to the left of the position ARIL shown in FIG. 24.Accordingly, the reference mark signal effect region 50ZRM-SE does notoverlap any reference mark signal receiver channel apertures and nosignificant signal is produced at the point 71″. As the reference mark50ZRM is displaced to the right, the reference mark signal effect region50ZRM-SE increasingly overlaps the REF1 reference mark signal receiverchannel aperture 2320R1 until a maximum amount of overlap is reached ata point 72″. As the reference mark 50ZRM continues to displace to theright, no further signal change is observed (e.g., because the dimensionLETOE(ZRMSE) is less than the dimension of the reference signal aperture2320R1) until the points 73″ and 73′″, which mark the left limit of asignal crossing region where the REF1 signal and the REF2 signalconverge (or diverge) to (or from) a common value. In the signalcrossing region, as the center line ZRMCSE of the zone grating referencemark signal effect region 50ZRMSE is displaced to the right of theposition (AR1L+0.5*LETOE(ZRMSE)), the signal REF1 begins to decrease asthe overlap between the reference mark signal effect region 50ZRMSE andthe REF1 reference mark signal receiver channel aperture 2320R1decreases. Because the reference mark signal receiver channel apertures2320R1 and 2320R2 have similar dimensions and zone grating referencemark 50ZRM is designed such that the dimension LETOE(ZRMSE) of thereference mark signal effect region 50ZRMSE is approximately equivalentto the dimension LCAR1CAR2 shown in shown in FIG. 24, the signal REF2simultaneously begins to increase at the point 73′″, as the overlapbetween the reference mark signal effect region 50ZRM-SE and the REF2reference mark signal receiver channel aperture 2320R2 increases. At apoint 74″, the center line ZRMCSE of the reference mark signal effectregion 50ZRMSE is symmetrically located between the REF1 reference marksignal receiver channel aperture 2320R1 and the REF2 reference marksignal receiver channel aperture 2320R2 (at the position shown in FIG.24) and the signals REF1 and REF2 are therefore nominally equal. Thebehavior of the signals REF1 and REF2 at the remaining points 75″, 75′″,76″ and 77″ may be understood by analogy with the previous explanation.The points 75″ and 75′″, analogous to the points 73″ and 73′″, mark theright limit of the signal crossing region.

In various exemplary embodiments, in order to provide a referenceposition along the measuring axis 82 in a robust manner, a referenceposition detection circuit may identify the position where the signalsREF1 and REF2 cross and are equal as the reference position, which isrepeatably related to the position of the zone grating reference mark50ZRM relative to the readhead of an integrated incremental signal andreference signal generating configuration as disclosed herein. Thereference signal generating configurations corresponding to the signalchart 70″ insures a robust signal crossing region that includes signalsthat nominally cross at a signal value approximately halfway betweentheir maximum and minimum values, as shown in FIG. 25.

FIG. 26 is a diagram showing a schematic side view illustrating variousaspects of the operation and design of an integrated scale grating andzone grating reference mark arrangement or structure 2600 usable invarious embodiments according to this invention, and a resulting zonegrating reference mark signal effect region 50ZRMSE. To illustrate anadvantageous relationship between resulting zone grating reference marksignal effect region 50ZRMSE and representative reference signalapertures 2620R1 and 2620R2, FIG. 26 also shows a side view of a maskelement 2661 which is similar to the mask element 2361 of FIG. 23, aswell as a plan view of that mask element 2661 rotated about the X-axisand maintained in alignment along the Z-axis direction.

The reference mark structure 2600 is analogous to the reference markstructure 2200 of FIG. 22, and similarly numbered elements may generallybe understood by analogy with previous description. For example, theraised elements E and the recessed elements G of the scale grating 80and the zone grating reference mark 50-ZRM′, may be configured accordingto principles described with reference to FIG. 22. Since designprinciples relating to the grating elements in each zone have alreadybeen described, the following description is primarily related to designprinciples relating to the overall dimensions of zone grating referencemark zones. For generality, the reference mark structure 2600 shown inillustrates 5 zones. However, it will be understood that fewer (or morezones) may be advantageous in various embodiments, as may be determinedby analysis or experiment. It should be appreciated FIG. 22 is not toscale. In particular, the horizontal scale is greatly exaggerated, forclarity. Thus, the angles shown in FIG. 22 are greatly exaggerated. Inpractice, in many embodiments, the optical path lengths to the variouszones shown in FIG. 22 may follow small angles (e.g., on the order of 1degree or less).

As previously outlined, to the extent that the widths of the raised andrecessed portions EZ and GZ are unmatched in the various zones of thezone grating reference mark 50ZRM, the unmatched portions willcontribute “zero order” reflected light. In terms of zone plateoperation, such light may be understood in terms of the well knownHuygens-Fresnel principle. Thus, it may be understood by one of ordinaryskill in the art having the benefit of this disclosure, that theunmatched portions of the widths of the raised and recessed portions EZand GZ may contribute light that behaves according to known zone plateprinciples. Thus, the zone grating reference mark 50ZRM may contributelight from various zones that constructively and destructively combineat certain locations such that it, in effect, provides a concentrated orfocused scale light component in the zone grating reference mark signaleffect region 50ZRM-SE at or proximate to the receiver plane 2660. Inparticular, the light from all zones should constructively interfere atzone grating reference mark signal effect region 50ZRM-SE.

FIG. 26 shows optical path lengths OPL1-OPLS which are the optical pathlengths from the light source 2680 and/or the zone grating referencemark signal effect region 50ZRM-SE to the representative locations forthe various zones ZONE1-ZONE 5. Also shown are the optical path lengthsOPL1OB, OPL2OB, OPL3OB, OPL4OB, and OPL5OB from the light source 2680and/or the zone grating reference mark signal effect region 50ZRM-SE tothe approximate outer boundaries of the zones ZONE1-ZONE5 (denoted Z′5),respectively. It may be seen that the recessed elements GZ provide theunmatched zone grating light in ZONE1, whereas the raised elements EZprovide the unmatched zone grating light in ZONE2. The height differencebetween the elements EZ and GZ may be one quarter of the wavelength λ ofthe light, as previously outlined. We consider this λ/4 separately fromthe optical path lengths OPL (e.g., OPL1, OPL2OB, etc.). To compensatefor this λ/4 path difference, and provide constructive interface betweenlight to and from ZONE1 and ZONE3, we can fulfill the condition:

2*(OPL2)=2*(OPL1−λ/4+λ/2)  (Eq. 15)

Where the first λ/4 term accounts for the aforementioned heightdifference and the λ/2 term provides constructive interference for theroundtrip light associated with the two zones. EQUATION 15 simplifiesto:

OPL2=OPL1+λ/4  (Eq. 16)

The boundary between ZONE1 and ZONE can be established at a locationthat provides an optical path length OPL1OB that is halfway between theoptical path lengths OPL1 and OPL2. That is;

OPL1OB=OPL1+λ/8  (Eq. 17)

By inspection of FIG. 26, it may be seen that:

0.5*ZONE 1 OBD=tan(cos⁻¹(OPL1/OPL1OB))*OPL1  (Eq. 18)

and that OPL1 is equal to the specified operating gap ZGAP. Therefore,substituting for OPL1OP from EQUATION 17, substituting ZGAP for OPL1:

ZONE1OBD=2*tan(cos⁻¹(ZGAP/(ZGAP+λ/8))*ZGAP  (Eq. 19)

It can be shown that EQUATION 19 may be expressed more generally for thenth ZONEn, as:

$\begin{matrix}{{{ZONEnOBD} \approx {\frac{1}{4}\sqrt{{\lambda^{2}*\left( {{2n} - 1} \right)^{2}} + {16\; \lambda*{ZGAP}*\left( {{2n} - 1} \right)}}} \approx \sqrt{\lambda*{ZGAP}*\left( {{2n} - 1} \right)}},{n = 1},2,{3\mspace{14mu} \ldots}} & \left( {{Eq}.\mspace{14mu} 20} \right)\end{matrix}$

For example, in one exemplary embodiment where ZGAP=5000 microns andL=0.650 microns, ZONE1OBD, ZONE2OBD, ZONE3OBD, ZONE4OBD and ZONE5OBD mayhave values of approximately 57 μm, 99 μm, 128 μm, 151 μm and 172 μmrespectively. The number of raised and recessed elements EZ and GZ ineach zone may be determined based on zone boundary locationsapproximately as outlined above, in combination with the grating pitchPG. In some embodiments, it is advantageous to make the raised orrecessed element that is closest to a zone boundary location (e.g., thatspans a zone boundary location) have a width that is 0.5*PG. Adjacentelements may then have widths corresponding to the desired duty cyclefor their respective zones, as previously outlined with reference toFIG. 22. As previously indicated, in various embodiments, a zone gratingreference mark need not have five zones, but may comprise more or fewerzones, which may satisfy the boundary locations outlined above. Itshould be appreciated that the foregoing boundary locations areexemplary only, and not limiting. In practice, the boundary locationsmay be altered somewhat based on analysis and/or experiment, to adjustthe effective intensity distribution and/or length LETOT(ZRMSE) of thezone grating signal effect region 50ZRM-SE, or to account for typicallyrelatively small additions to optical path lengths from non-zero Y-axispositions of REF1 and REF2 apertures, to provide desirable referencesignals REF1 and REF2. For example, in some embodiments, the values ofZONEnOBD may fall in a range of 0.8 to 1.2 times the nominal values forZONEnOBD given by EQUATION 20.

FIGS. 27A and 27B are illustrations 27000A and 27000B, respectively,showing alternative aperture mask configurations usable in place ofportions of the aperture mask configuration of the mask element 2361shown in FIG. 24. In particular, FIGS. 27A and 27B show alternativeaperture mask configurations for those portions of a mask elementassociated with the generating the reference mark signals REF1 and REF2.Elements numbered with analogous numbers (e.g., similar numericalsuffixes) in FIGS. 27A, 27B and FIG. 24 may have similar designprinciples and operation, and may be understood by analogy unlessotherwise indicated below. It will be understood that mask elementportions not explicitly shown in FIGS. 27A and 27B may be similar oridentical to those illustrated for the mask element 2361 in FIG. 24, orotherwise according to principles taught in this application or theincorporated references.

In comparison to the configuration of the blocking mask portion 2320BR1,reference signal aperture 2320R1, blocking mask portion 23420BR2 andreference signal aperture 2320R2 shown in FIG. 24, the primarydifference in FIG. 27A is that the blocking mask portion 2720BR1′,reference signal aperture 2720R1, blocking mask portion 2720BR2 andreference signal aperture 2720R2, provide fewer left and rightzigzagging aperture edge segments along the aperture edges that extendtransverse to the measuring axis direction. The previously outlineddesign principles (e.g., related to the dimensions PAET and PAW) maystill be applied.

FIG. 27B illustrates a configuration which may be understood by analogyto the previous description of FIG. 24 and FIG. 27A. In particular, theblocking mask portion 2720BR1′, reference signal aperture 2720R1′,blocking mask portion 2720BR2′ and reference signal aperture 2720R2′,provide left and right aperture edges that extend transverse to themeasuring axis direction and those transverse aperture edges are angledrelative to the Y-axis such that each transverse aperture edge spans aprimary aperture edge transition dimension PAET extending from anadjacent aperture boundary (e.g., the boundary indicated by the dashedline AR2R) along the direction of the measuring axis 82 toward theopposite edge of the aperture. In comparison to the configuration ofFIG. 27A, the primary difference is that the apertures shown in FIG. 27Bhave the character of a “rotated rectangle,” or “rotated edges” ratherthan having zigzag edges. Nevertheless, the apertures shown in FIG. 27Bhave analogous dimensions PAET and PAW that may fulfill the designprinciples previously outlined with reference for FIG. 27A. Based on theforegoing examples of FIGS. 27A and 27B, it will be appreciated thatapertures edges may alternatively have segments that are curved (ratherthan straight and angled), or that meander in some other manner, over aprimary aperture edge transition dimension PAET, and such designs mayalso fulfill the design principles previously outlined with referencefor FIG. 27A. Therefore, it will be appreciated that the foregoingexamples are illustrative only, and not limiting. It should also beappreciated that apertures designed according to the principles outlinedwith reference to FIGS. 27A and 27B may generally be used in variousother embodiments of the invention (e.g., in place of the referencesignal apertures shown in FIG. 24).

FIG. 28 is an isometric view schematically showing various aspects of aportion 28000′ of an integrated incremental signal and reference signalgenerating configuration usable in various embodiments according to thisinvention, which uses an integrated scale grating and zone gratingreference mark arrangement that includes two zone grating reference markportions (not shown). The design and operation of the portion 28000′ isin many respects similar to that of the portion 23000′ of FIG. 24, andsimilarly numbered elements in the 23XX and 28XX series of numbers(e.g., with similar numerical suffixes such as the elements 2320R2 and2820R2) may be analogous or identical in form and/or operation, exceptas otherwise indicated below. Generally, the design and operation of theportion 28000′ may be understood based on the previous description ofthe portion 23000′ and the integrated signal generating configuration23000 and/or design principles taught elsewhere in this disclosure or inthe incorporated references. Therefore, only the significant differencesbetween the operation of the portions 23000′ and 28000′ are describedbelow.

FIG. 28 shows the portion 28000′ including the integrated readheadoptical fiber arrangement 2800, the reference mark signal effect region50ZRM-SE, and a mask element 2861. The primary difference between theportions 23000′ and 28000′ is that the optical fiber arrangements 2300and 2800 have a different rotational orientation in the XY plane. In theoptical fiber arrangement 2800, receiver fibers 2890R1 and 2890R2, whichare separated along the X-axis direction, provide the reference signalsREF1 and REF2. In addition, the reference mark signal effect region50ZRM-SE includes two signal effect sub-regions or portions 50ZRM1-SEand 50ZRM2-SE, having individual dimensions LSEG, and providing aninterior edge-to-edge dimension LETOE(ZRMSE). It will be appreciatedthat a corresponding zone grating reference mark, hereby designated50ZRM, (not shown) including two zone grating sub-portions or referencemark portions designated 50ZRM1 and 50ZRM2 provides the two separatedzone grating reference mark signal effect sub-regions or portions50ZRM1-SE and 50ZRM2-SE according to previously outlined principles.Although in the particular embodiment shown in FIG. 28, the dimensionLETOE(ZRMSE) corresponds to the distance between the interior boundariesof the two signal effect sub-regions 50ZRM1-SE and 50ZRM2-SE, it shouldbe appreciated that in an alternative embodiment, the zone gratingreference mark may be designed for a relationship between the signaleffect sub-regions or portions 50ZRM1-SE and 50ZRM2-SE such that thedistance between their exterior boundaries (rather than their interiorboundaries) corresponds to the same dimension LETOE(ZRMSE). In eithercase, it should be appreciated that a reference mark may be structuredto include two reference mark sub-regions or portions that areseparated, producing the type of signal effect sub-regions or portions50ZRM1-SE and 50ZRM2-SE shown in FIG. 28, which may be used with anintegrated readhead optical fiber arrangement such as that shown in FIG.28.

While this invention has been described in conjunction with theexemplary embodiments outlined above, it is evident that the embodimentsand design factors described above are indicative of additionalalternative embodiments, modifications and variations, as will beapparent to those skilled in the art. Accordingly, the embodiments ofthe invention, as set forth above, are intended to be illustrative, notlimiting. Various changes may be made without departing from the spiritand scope of the invention.

1. A fiber optic readhead and scale arrangement comprising a fiber opticreference signal generating configuration usable to provide anindication of reference position between two members that move relativeto one another along a measuring axis direction, the fiber opticreference signal generating configuration comprising: at least a portionof a first fiber optic readhead, the at least a portion of a first fiberoptic readhead comprising a light source that outputs a first divergingsource light, and at least first and second fiber optic reference marksignal receiver channels that are configured to provide respectivereference mark signals, the first fiber optic reference mark signalreceiver channel comprising a first reference mark signal receiverchannel optical fiber and a first reference mark signal receiver channelaperture, and the second fiber optic reference mark signal receiverchannel comprising a second reference mark signal receiver channeloptical fiber and a second reference mark signal receiver channelaperture; and a first scale track extending along the measuring axisdirection on a scale member, the first scale track configured to reflectthe first diverging source light to provide scale light to the firstfiber optic readhead, the first scale track comprising: a first type oftrack portion extending over a displacement measuring range along themeasuring axis direction and contributing to an incremental signal lightcomponent of the scale light when illuminated by the diverging sourcelight, the first type of track portion comprising a reflective periodicphase grating having a grating pitch PG and configured to suppresszero-order reflected light and provide a spatially periodic intensitypattern in the incremental signal light component of the scale lightproximate to a receiving plane of the first fiber optic readhead thereflective periodic phase grating comprising: raised scale elements thatare nominally centered at corresponding raised grating element nominallocations, wherein the raised grating element nominal locations arespaced apart from one another by the grating pitch PG along themeasuring axis direction, and recessed scale elements that are centeredat corresponding recessed grating element nominal locations, therecessed grating element nominal locations spaced apart from one anotherby the grating pitch PG along the measuring axis direction, and offsetfrom the raised grating element nominal locations by one half thegrating pitch PG along the measuring axis direction; and a zone gratingreference mark comprising at least one zone grating reference markportion located within the first type of track portion and contributingto a reference mark signal light component of the scale light whenilluminated by the diverging source light, each zone grating referencemark portion configured such that the reference mark signal lightcomponent of the scale light includes reflected light that isconcentrated to provide an elevated reference mark signal lightintensity across a corresponding reference mark signal effect regionportion proximate to the receiving plane of the first fiber opticreadhead when the first fiber optic readhead is operably positionedrelative to the first scale track, wherein the zone grating referencemark and its at least one corresponding reference mark signal effectregion have a repeatable relationship along the measuring axisdirection, and the first and second reference mark signal receiverchannel apertures receive detectably different amounts of scale lightdepending on their proximity with the at least one zone gratingreference mark portion and its at least one corresponding reference marksignal effect region portion, wherein: the raised grating elementnominal locations and the recessed grating element nominal locations aredefined to continue from the first type of track portion along themeasuring axis direction through the at least one zone grating referencemark portion; the at least one zone grating reference mark portioncomprises a reflective phase grating comprising a plurality of raisedzone grating elements that are each arranged to span a correspondingraised grating element nominal location, and a plurality of recessedzone grating elements that are each arranged to span a correspondingrecessed grating element nominal location; the at least one zone gratingreference mark portion is configured to include at least a first zone,and a second zone that is farther from the reference mark signal effectregion than the first zone; the first zone comprises a first set of theraised zone grating elements and a first set of the recessed first zonegrating elements, wherein: the members of the first set of raised zonegrating elements have an average raised element width W1E along themeasuring axis direction, the members of the first set of recessed zonegrating elements have an average recessed element width W1G along themeasuring axis direction, and in the first zone, a longer one of theaverage raised element width W1E and the average recessed element widthW1G is greater than (0.5*PG), and shorter one of the average raisedelement width W1E and the average recessed element width W1G is lessthan (0.5*PG); the second zone comprises a second set of the raised zonegrating elements and a second set of the recessed zone grating elements,wherein: the members of the second set of raised zone grating elementshave an average raised element width W2E along the measuring axisdirection, the members of the second set of recessed zone gratingelements have an average recessed element width W2G along the measuringaxis direction, in the second zone, a longer one of the average raisedelement width W2E and the average recessed element width W2G is greaterthan (0.5*PG), and shorter one of the average raised element width W2Eand the average recessed element width W2G is less than (0.5*PG), and ifW1E>W1G in the first zone, then W2G>W2E in the second zone, and ifW1E<W1G in the first zone, then W2G<W2E in the second zone; the firstand second reference mark signal receiver channel apertures areconfigured such that their aperture boundaries that are closest to oneanother along the measuring axis direction are separated by a dimensionAR12SEP along the measuring axis direction, and their apertureboundaries that are farthest from one another along the measuring axisdirection span a total aperture span dimension AR12SPAN along themeasuring axis direction, when the first fiber optic readhead isoperably positioned relative to the first scale track; the at least onereference mark portion is configured to such that two signal effectregion boundaries of its at least one corresponding reference marksignal effect region portion are separated by an edge-to-edge dimensionLETOEZRMSE along the measuring axis direction; the at least onereference mark portion and the first and second fiber optic referencemark signal receiver channel apertures are configured such thatAR12SEP<LETOEZRMSE<AR12SPAN; and when the first fiber optic readhead isoperably positioned relative to the first scale track, the at leastfirst and second fiber optic reference mark signal receiver channelsinput portions of the scale light through their apertures and transmitthose input portions of the scale light to provide their respectivereference mark signals, which indicate of a reference position within asignal crossing region proximate to the zone grating reference mark. 2.The fiber optic readhead and scale arrangement of claim 1, wherein theelevated reference mark signal light intensity across a correspondingreference mark signal effect region portion comprises an elevatedintensity distribution across the corresponding reference mark signaleffect region portion, and each of the two signal effect regionboundaries are defined to fall within a range where the intensitycontribution of the reference mark signal light component of the scalelight is at most 80% of its maximum contribution in the elevatedintensity distribution and at least 20% of its maximum contribution inthe elevated intensity distribution.
 3. The fiber optic readhead andscale arrangement of claim 1, wherein: the first diverging source lighthas an average wavelength λ; the fiber optic readhead and scalearrangement has a specified operating gap ZGAP between the receivingplane of the of the first fiber optic readhead and a plane of the firstscale track; the at least one zone grating reference mark portioncomprises at least two zones; and the nth zone of the at least one zonegrating reference mark portion has an outer boundary dimension along themeasuring axis direction that spans at least 0.8 times, and at most 1.2times, a nominal outer boundary dimension ZONEnOBD for the nth zone,where${ZONEnOBD} = {\frac{1}{4}{\sqrt{{\lambda^{2}*\left( {{2n} - 1} \right)^{2}} + {16\; \lambda*{ZGAP}*\left( {{2n} - 1} \right)}}.}}$4. The fiber optic readhead and scale arrangement of claim 1, whereinthe widths WEZ and WGZ vary from PG/2 by at most +/−25% of PG and atleast +/−5% of PG.
 5. The fiber optic readhead and scale arrangement ofclaim 4, wherein the widths WEZ and WGZ vary from PG/2 by at most +/−20%of PG and at least +/−10% of PG.
 6. The fiber optic readhead and scalearrangement of claim 1, wherein a raised or recessed zone gratingelement that is closest to a boundary between adjacent zones of the zonegrating reference mark has a width that is approximately 0.5*PG.
 7. Thefiber optic readhead and scale arrangement of claim 1, wherein: thefirst and second reference mark signal receiver channel apertures eachcomprise a respective aperture mask located proximate to the end oftheir reference mark signal receiver channel optical fiber to cover aportion of its light carrying core area, and the respective aperturemask includes at least a first transverse aperture edge that includes aportion that is not perpendicular to the measuring axis direction andspans an aperture edge transition dimension PAET extending from anadjacent aperture boundary along the measuring axis direction toward anopposite transverse aperture edge.
 8. The fiber optic readhead and scalearrangement of claim 7, wherein: the spatially periodic intensitypattern comprises interference fringes having an interference fringepitch FP at a plane of the respective aperture mask and the apertureedge transition dimension PAET spans at least one interference fringepitch FP at the plane of the respective aperture mask, when the firstfiber optic readhead is operably positioned relative to the first scaletrack.
 9. The fiber optic readhead and scale arrangement of claim 8,wherein the aperture edge transition dimension PAET is nominally equalto an integer number of interference fringe pitches FP at the plane ofthe respective aperture mask.
 10. The fiber optic readhead and scalearrangement of claim 8, wherein the opposite transverse aperture edgecomprises a second transverse aperture edge of the respective aperturemask, and the first and second aperture edges define an aperture widthdimension PAW along the measuring axis direction between them, thedimension PAW defined at each location along a direction transverse tothe measuring axis direction.
 11. The fiber optic readhead and scalearrangement of claim 10, wherein the dimension PAW is nominally equal toan integer number of interference fringe pitches FP at the plane of therespective aperture mask, at least along a majority of the length of thefirst and second aperture edges, and the aperture edge transitiondimension PAET is nominally equal to an integer number of interferencefringe pitches FP at the plane of the respective aperture mask.
 12. Thefiber optic readhead and scale arrangement of claim 7, wherein the fiberoptic reference signal generating configuration is configured accordingto one of a single-portion zone grating reference mark configuration,and a two-subportion zone grating reference mark configuration, wherein:in the single-portion zone grating reference mark configuration the atleast one zone grating reference mark portion consists of a single zonegrating reference mark portion, and the single zone grating referencemark portion is configured to provides a zone grating reference marksignal effect region that has outer boundaries that are separated alongthe measuring axis direction by the LETOEZRMSE; and in thetwo-subportion zone grating reference mark configuration the at leastone zone grating reference mark portion comprises first and second zonegrating reference mark sub-portions, configured according to one ofconfiguration A and configuration B, wherein, in configuration A thefirst and second reference mark sub-portions are configured to providecorresponding zone grating reference mark signal effect region portionswherein the outer boundaries of the zone grating reference mark signaleffect region portions that are closest to one another along themeasuring axis direction are separated by the edge-to-edge dimensionLETOEZRMSE, and in configuration B the first and second reference marksub-portions are configured to provide corresponding zone gratingreference mark signal effect region portions wherein the outerboundaries of the zone grating reference mark signal effect regionportions that are farthest from one another along the measuring axisdirection are separated by the edge-to-edge dimension LETOEZRMSE. 13.The fiber optic readhead and scale arrangement of claim 12, wherein: thefiber optic reference signal generating configuration is configuredaccording to the single-portion reference mark configuration; theopposite transverse aperture edge comprises a second transverse apertureedge of the respective aperture mask, and the first and second apertureedges define an aperture width dimension PAW along the measuring axisdirection between them, wherein the aperture dimension PAW is defined ateach location along a direction transverse to the measuring axisdirection and is nominally equal to an integer number of interferencefringe pitches FP at the plane of the respective aperture mask, at leastalong a majority of the length of the first and second aperture edges;and the aperture edge transition dimension PAET is nominally equal to aninteger number of interference fringe pitches FP at the plane of therespective aperture mask.
 14. The fiber optic readhead and scalearrangement of claim 1, wherein the first diverging source light isspatially coherent and monochromatic.
 15. The fiber optic readhead andscale arrangement of claim 1, wherein: the spatially periodic intensitypattern comprises interference fringes having an interference fringepitch FP; and the first fiber optic readhead further comprises aplurality of respective fiber optic incremental measurement signalreceiver channels that are configured to provide respective spatiallyperiodic incremental measurement signals, each respective fiber opticincremental measurement signal receiver channel comprising a respectiveincremental measurement signal receiver channel optical fiber and arespective incremental measurement signal receiver channel spatial phasemask portion arranged proximate to an end of that optical fiber, therespective incremental measurement signal receiver channel spatial phasemask portion having a respective spatial phase and having light-blockingelements arranged at a pitch that is operable for spatially filteringthe spatially periodic intensity pattern included in the scale lightreflected from the first scale track, wherein: the fiber optic readheadand scale arrangement is configured such that the respective referencemark signals provide the indication of the reference position within thesignal crossing region proximate to the at least one reference markportion, with a position repeatability that less than +/−one-half periodof the spatially periodic incremental measurement signals.
 16. The fiberoptic readhead and scale arrangement of claim 15, wherein: the lightsource comprises the end of a source optical fiber; the at least firstand second reference mark signal receiver channel optical fibers andeach of the respective incremental measurement signal receiver channeloptical fibers are located parallel to and proximate to the sourceoptical fiber; and all optical fibers located within the first fiberoptic readhead, including the source optical fiber, the at least firstand second reference mark signal receiver channel optical fibers, andeach of the respective incremental measurement signal receiver channeloptical fibers, are located within a cylindrical volume having adiameter of at most 1.5 millimeters.